unstable/text-input: Add a .process_keys request

This request allows clients to solve the egg-and-chicken problem coming from type-to-search UIs, where events already sent as wl_keyboard.key events to the client trigger text_input being enabled, thus those events should be reprocessed by the IM to obtain the expected output.

It allows passing a wl_keyboard.key serial, so that compositors may keep a small buffer of recent key events, and replay those to the input method. Clients doing this request should ignore the effects of wl_keyboard.key events that might be already queued up, until the text_input acknowledged the change.

Signed-off-by: Carlos Garnacho carlosg@gnome.org