Add KeyboardHandle::advertise_modifier_state

[axelkar]

Fixes #1018

[axelkar]

This is important for keysym input emulation where xkbcommon just gives us possible modifier masks and keycodes.

https://github.com/rust-x-bindings/xkbcommon-rs/blob/cb449998d8a3de375d492fb7ec015b2925f38ddb/examples/how-to-type.rs#L120-L146

The keyboard middleware should probably be structured a little differently. It's all in one giant file, and ModifiersState can contain duplicate and outdated information. The serialized field and the other fields in ModifiersState aren't guaranteed to be updated in sync.

It's also not immediately obvious, as a consumer, where the data moves and what structs I can access. (KeyboardHandle, KeyboardInnerHandle, KeybordGrab, KeyboardTarget).

It's also not immediately obvious that I should modify the modifier state with KeyboardHandle::set_modifier_state rather than KeyboardHandle::with_xkb_state and xkb::State::update_mask to get integration with the "high-level/lossy modifier state" as I've been calling it.

[axelkar]

Rebased on master