Auto merge of #20480 - kwonoj:refactor-file-manager, r=paulrouget

refactor(net): removes direct ui invocation from filemgr thread

<!-- Please describe your changes on the following line: -->
- relates to https://github.com/servo/servo/issues/20428.

This PR tries to refactor `net::filemanager_thread` implementation, removes direct ui-related code invocation but ask constellation. I believe overall organization might need to be refactored still as I took my own liberty to wire up dots and dots between components, which could be non-recommended practices.

Probably point of review / need to be updated are

1. Communication between components
Currently it's wired as like below:
```
+----------------+                    +---------------+              +---------+
|                |  constellationMsg  |               | embedderMsg  |         |
| filemgr_thread +------------------->+ constellation +------------->+ browser |
|                |                    |               |              |         |
+-----+----------+                    +---------------+              +----+----+
      ^                                                                   |
      +-------------------------------------------------------------------+
                                filelist: Vec(String)
```
- is this feasible approach?
- does organization of message / fn (where to put consteallation / embedder msg & fns) are legit?

2. Removal of `filemanger_thread::UIProvider`
- As filemanager_thread no longer need to aware actual ui, this PR removes `UIProvider` completely and let invoke tinyfiledialog directly in message listener. Maybe UIProvider itself still being needed?

3. Overall fn organization
- To reduce duplicated code it takes single msg with boolean flag to distinguish selecting multiple files, may feasible / or better to create explicit paths between two.

4. Invoking tfd in a separate thread
- This was mainly to align behavior to previous implentation, where tfd was invoked inside of filemanager_thread so does not block main. It may possibly just let block instead.

and of course, a lot of other codes need to follow better practices.

---
<!-- 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 #20428 (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____
- Manually verified file picker `<input type=files>` can pick up files.

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- 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="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/20480)
<!-- Reviewable:end -->

Cargo.lock

@@ -1885,6 +1885,7 @@ version = "0.0.1"
 dependencies = [
  "base64 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "brotli 1.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "compositing 0.0.1",
  "cookie 0.10.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "devtools_traits 0.0.1",
  "embedder_traits 0.0.1",
@@ -1914,7 +1915,6 @@ dependencies = [
  "servo_url 0.0.1",
  "threadpool 1.3.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "time 0.1.37 (registry+https://github.com/rust-lang/crates.io-index)",
- "tinyfiledialogs 3.3.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "unicase 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "url 1.6.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "uuid 0.6.2 (registry+https://github.com/rust-lang/crates.io-index)",