Implement <meta name=viewport> handling
Translate <meta name=viewport> as according to [CSS Device Adaption § 9](http://dev.w3.org/csswg/css-device-adapt/#viewport-meta)
Note: as the PR currently stands, handling `<meta name=viewport>` elements always occurs. This is probably not desired for some contexts (e.g. desktop), but I'm unsure of how to conditionally handle elements based on that.
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Refactor FormControl trait
The trait is now implemented for HTMLFooElement instead of
&HTMLFooElement and does no longer require an impl body.
Suggested by @Manishearth
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Add form getters for additional elements; update test expectations
This adds form getters for fieldset, label, object, output, select and
textarea elements.
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Properly consider radio button groups
Radio button groups with missing or empty names are always the only element of their respective radio button group.
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webgl: Support texImage2D with a canvas as an argument
This involved some refactoring of the 2d context code, which lead to some more test passed there.
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This allows both boolean and string-type preferences. It
also implements a system where prefs that are read from a
configuration file can be reset back to their initial value,
which is useful in a number of cases e.g. when running tests
to ensure that each test starts with the same values for
the prefs.
This fixes a crash resulting from a race between loading an initial
document and navigating to a subsequent document. If the navigation
happens before the initial document has had a chance to create its
root page, we crash trying to unwrap a None. Note that the are likely
further similar timing issues with more complex sequences of navigation
and history manipulation.
Implement location.reload()
This is a naive implementation of `window.location.reload()`.
I'd appreciate any feedback.
I was wondering if it'd be better to implement `ConstellationMsg::Reload` instead of using `load_url`.
Also, what kind of test should I write?
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