Hardware Keyboard: Linux (GTK)

[dkwingsmt]

Description

This PR contains the Linux (GTK) changes for the Hardware Keyboard project.

Classes has been adjusted as follows:

Before	After	Explain
FlKeyEventPlugin	FlKeyboardManager	Handles redispatching and detects redispatched events.
	FlKeyChannelResponder	Sends raw events through method channel and collect responses.
-	FlKeyEmbedderResponder	Converts raw events to FlutterKeyEvent, sends them through embedder API and collect responses.

Related Issues

• Umbrella: ☂️ Device-specific keyboard events should happen in the embedder flutter#44918

Tests

I added the following tests:

• A number of tests for FlKeyboardManager
• A number of tests for FlKeyEmbedderResponder

Migrated tests of FlKeyEventPlugin to FlKeyChannelResponder.

Checklist

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• I updated/added relevant documentation.
• All existing and new tests are passing.
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Reviewer Checklist

• I have submitted any presubmit flakes in this PR using the engine presubmit flakes form before re-triggering the failure.

Breaking Change

Did any tests fail when you ran them? Please read handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.
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[dkwingsmt]

@gspencergoog Can you take another look?
For your convenience, this is the since after your last review. The changes by this PR are everything under the linux folder (including fl_settings_plugin.cc).
https://github.com/flutter/engine/compare/3ecc134bc68f96b01e2dd3fd921e4c4406db454f..f36473a38a9e3694b62920accb5048e7fe7f65a6

[gspencergoog]

@dkwingsmt I think that link compares the wrong revisions, unless you've been VERY busy :-) (it currently shows ~5000 lines of difference in 115 files for me). No worries, though, I'll just go through the code.

[dkwingsmt]

@gspencergoog That's because I've updated with master since your last review, and that's what I mean by "the changes by this PR are everything under the linux folder". (So everything out of it are from the master)

Edit: Ok I just learned that Github comes with the "PR compare" feature so here we go:
https://github.com/flutter/engine/pull/23467/files/3ecc134bc68f96b01e2dd3fd921e4c4406db454f..f36473a38a9e3694b62920accb5048e7fe7f65a6

[gspencergoog]

To be honest, I have looked this over a number of times, but there's so much code here, it's hard to be sure that things are all "correct". In general, it looks good though.

[gspencergoog]

Remove commented out code.

[dkwingsmt]

@gspencergoog Thanks for the review!