With the policy being disentangled from application, it becomes testable.
This prepares for moving the entire visibility mechanism to the new class and taking away more pieces of ServerContextService.
In addition, this is a good warmup before trying to implement sizing policy.
Control and Alt are special in that they aren't expected to switch levels, and so don't need to change what characters are output.
Use in layouts by adding `modifier: Control` or `modifier: Alt` in place of `text: "foo"`.
The latching of the modifier will force the keyboard to emit raw key presses and prevent it from outputting text.
Erasing with zwp_text_input_v3 version 1 requires bytes, and bytes require get_surrounding_text. That, however, is optional. That's a mistake in protocol design.
Easiest to drop this until the mess is solved on the protocol side.
This fixes some rendering things which would happen with multiple state-sharing buttons. It also removes some interfaces exposing rows, views, layouts, and buttons, bringing the code closer to removing them from the FFI entirely.
