Implements focus traversal for desktop platforms, including re-implementing the existing focus manager and focus tree.
This implements a Focus widget that can be put into a widget tree to allow input focus to be given to a particular part of a widget tree.
It incorporates with the existing FocusScope and FocusNode infrastructure, and has minimal breakage to the API, although FocusScope.reparentIfNeeded is removed, replaced by a call to FocusAttachment.reparent(), so this is a breaking change:
FocusScopeNodes must now be attached to the focus tree using FocusScopeNode.attach, which takes a context and an optional onKey callback, and returns a FocusAttachment that should be kept by the widget that hosts the FocusScopeNode. This is necessary because of the need to make sure that the focus tree reflects the widget hierarchy.
Callers that used to call FocusScope(context).reparentIfNeeded in their build method will call reparent on a FocusAttachment instead, which they will obtain by calling FocusScopeNode.attach in their initState method. Widgets that own FocusNodes will need to call dispose on the focus node in their dispose method.
Addresses #11344, #1608, #13264, and #1678Fixes#30084Fixes#26704
This makes Android raw key event handling use the "character" data coming from the engine properly, and gets rid of the "toLowerCase" hack that I was using to normalize logical key events, in favor of using the new "plainCodePoint" that has the unmodified code point (the code point as if no modifier keys were pressed).
This adds support for logical and physical key information inside of RawKeyEvent. This allows developers to differentiate keys in a platform-agnostic way. They are able to tell the physical location of a key (PhysicalKeyboardKey) and a logical meaning of the key (LogicalKeyboardKey), as well as get notified of the character generated by the keypress. All of which is useful for handling keyboard shortcuts.
This PR builds on the previous PR (#27620) which generated the key code mappings and definitions.
This adds some functions to the interface for RawKeyEventData and all subclasses that allow the recipient of an event to determine which modifier keys are currently being pressed without needing to know the specific modifier bitmasks for the platform.
Also adds constants for the modifier bitmasks for each platform, for completeness (and because I needed them anyhow to implement the above).
Added tests for the RawKeyEventData subclasses, and modified the raw_keyboard manual test app to show modifier keys being pressed. I also separated the different platform-specific subclasses into separate files.
Fixes#26155.
