Improve key event handling for TV remote controls

[swift-kim]

Fixes https://github.com/flutter-tizen/flutter-tizen/issues/319. Substitutes the previous change https://github.com/flutter-tizen/engine/pull/232.

Changes

• Let some special system keys (including XF86AudioPlayPause) be redefinable by the app. ecore_wl2_window_keygrab_set and eext_win_keygrab_set were used to override the system's default behavior.
• Add efl-extension dependency (for eext_win_keygrab_set).
• Implement character to UTF32 code point conversion to support RawKeyEvent.character.
• Make KeyEventHandler::OnKey return ECORE_CALLBACK_DONE (instead of ECORE_CALLBACK_PASS_ON) by default.
• Update the value of modifierMeta (1 << 28 to 1 << 26).

Cleanups

• Use consistent words in variable names and comments (e.g. physical key code → scan code).
• Rename variables to clarify their meaning (e.g. key → event).
• Make the structures of key_event_handler.cc and touch_event_handler.cc analogous to each other.

To test this PR (details):

$ TIZEN_ENGINE_GITHUB_RUN_ID=1804716824 flutter-tizen precache --tizen
$ flutter-tizen run --verbose-system-logs

[swift-kim]

I decided not to apply https://github.com/flutter-tizen/engine/issues/221 right now because it adds too much complexity without much benefit.

We still need to implement it in the near future in order to enable more key mapping (currently only keys that are listed in kGtkToLogicalKey are supported).

[bbrto21]

After applying this PR, I confirmed that the "Not available" popup no longer appears on my TV with devel board

[Aliaksandr-Tuzik]

@swift-kim, is there a reason it goes into Flutter 2.10 and not in 2.8? Can this be done for 2.8 as well?

[swift-kim]

@AliaksandrTuzik We usually support only one Flutter version at a time (i.e. the flutter-tizen tool follows a linear versioning policy) to reduce maintenance workload and do not provide updates for older versions. However as https://github.com/flutter-tizen/flutter-tizen/pull/326 has not been merged yet, we can release this patch for Flutter 2.8 first and then finish the migration to Flutter 2.10, if you want. How do you think?

[Aliaksandr-Tuzik]

@AliaksandrTuzik We usually support only one Flutter version at a time (i.e. the flutter-tizen tool follows a linear versioning policy) to reduce maintenance workload and do not provide updates for older versions. However as flutter-tizen/flutter-tizen#326 has not been merged yet, we can release this patch for Flutter 2.8 first and then finish the migration to Flutter 2.10, if you want. How do you think?

That would be great!