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

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

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- 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.

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- [x] `./mach build -d` does not report any errors
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- [x] These changes fix #20428 (github issue number if applicable).

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- Manually verified file picker `<input type=files>` can pick up files.

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