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.