Properly track keymap when virtual keyboard is in use

[kchibisov]

• Have dedicated input types for both normal input and
  Virtual keyboard. Each type should track its keymap
  and modifiers state separatelly. They should also
  register into some global type that they were
  the last to send keys for the given client.

• When sending input events for the client,
  smithay should compare old != new source
  and resend keymap with the modifiers being
  active on that keymap, and then forward the
  state.

The tracking should be per-seat, not per-client
since seat can focus only one client at a time
and virtual keyboard just improves on normal
keyboard.

[kchibisov]

And to clarify, that is only when sending input to clients, like you decided to type not on a virtual keyboard but on a regular one, and you don't have IM grab going on. But that could be naturally determined due to a fact that virtual keyboard can't type into grab.

The smithay should also never use keymap from the virtual_keyboard for itself.