Fix Issue 12343: Add pref-gated methods and test for activatable element
<!-- Please describe your changes on the following line: -->
Add methods to activatable elements; gated by a preference, for use in servo-specific WPT tests.
I cargo-culted over the "redundant check" from #12322, I don't really understand the possible exploit.
Obvious flaw is the copypasta for each element. I understand that the webidl codegen will produce a trait for ActivatableElement, but I can't see how I can provide a default implementation for this? I guess otherwise could be an improvement to have each element just delegate to an Enter.../Exit... method on the Activatable trait?
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#12343 (github issue number if applicable).
<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
…states
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12703)
<!-- Reviewable:end -->
Issue 8719: Add basic support for :active selector
<!-- Please describe your changes on the following line: -->
Added toggling of active state for element and parents on mousedown/mouseup. Active state is removed when mouseout. (hover)
- As with my other PR i'm struggling a bit with the automated testing. I've added a manual test case and found quirks-mode/active-and-hover-manual.html which - aside from also being a manual test, is functional in Firefox but does not render correctly in Servo.
- Not implemented: In Firefox, behaviour differs with a <!DOCTYPE HTML> and an anchor does not lose it's activation on mouseout; whereas a button does.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#8719 (github issue number if applicable).
<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/11781)
<!-- Reviewable:end -->
csstext
- [x] These changes fix#4431.
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy --faster` reports one error for the `css_properties_accessors!` macro not having a spec link
```
$ git log -1 --format=oneline
91fb9bf1d3 fixup! implement cssText
$ ./mach build -d
Compiling style v0.0.1 (file:///Users/greg/servo/components/style)
Compiling gfx v0.0.1 (file:///Users/greg/servo/components/gfx)
Compiling script v0.0.1 (file:///Users/greg/servo/components/script)
Compiling layout_traits v0.0.1 (file:///Users/greg/servo/components/layout_traits)
Compiling compositing v0.0.1 (file:///Users/greg/servo/components/compositing)
Compiling glutin_app v0.0.1 (file:///Users/greg/servo/ports/glutin)
Compiling constellation v0.0.1 (file:///Users/greg/servo/components/constellation)
Compiling layout v0.0.1 (file:///Users/greg/servo/components/layout)
Compiling servo v0.0.1 (file:///Users/greg/servo/components/servo)
Build completed in 0:05:11.475584
$ ./mach test-tidy --faster
Checking files for tidiness...
./components/script/dom/cssstyledeclaration.rs:386: method declared in webidl is missing a comment with a specification link
Progress: 100% (12/12)
```
- [x] There are tests for these changes. More CSSOM tests pass, but others fail that probably shouldn't:
* `./mach test-css tests/wpt/css-tests/cssom-1_dev/html/index-002.htm` and a bunch of the other tests in `/css-tests/cssom-1_dev/html/ crash when run individually
* `./mach test-css tests/wpt/css-tests/cssom-1_dev/html/cssom-cssText-serialize.htm` fails to strip a trailing semicolon (`left: 10px` vs `left: 10px;`)
* `./mach test-css tests/wpt/css-tests/cssom-1_dev/html/index-001.htm` shared shorthand values aren't coalesced (`margin: 20px` vs. `margin: 20px 20px 20px 20px`)
* `./mach test-css tests/wpt/css-tests/cssom-1_dev/html/cssstyledeclaration-csstext.htm` also crashes and fails for `color: red;` vs. `color: RED;`, preserving declaration insertion order, whitespace in the value, and setting an unknown style property.
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/11428)
<!-- Reviewable:end -->
