[Accessibility] Need to be able to register for key events globally

[Piñeiro]

GNOME Assistive Technologies need to be able to listen to key events
globally and have the possibility of consuming them. Example use
cases:

* Orca's presentation of navigation (Events not consumed)
   - Right Arrow: Speak character moved to (right of cursor)
   - Shift Right Arrow: Speak character selected (left of cursor)
   - Down Arrow: Speak next line
   - Etc.
* Orca's navigational commands (Events consumed)
   - H/Shift + H: Move amongst headings
   - NumPad 8: Speak the current line
   - NumPad 5: Speak the current word
   - NumPad 2: Speak the current character
   - Etc.

Current solution: The Orca screen reader calls AT-SPI2's
atspi_register_keystroke_listener(). AT-SPI2 then notifies Orca of key
events it receives from the toolkit implementation of ATK method
atk_add_key_event_listener(). Applications then have to wait for Orca
to consume the event or not. This requires two DBUS messages. Toolkit
authors want to abolish this. That's fine, *if* we have an
alternative. Do we?


-- 
Alejandro Piñeiro Iglesias (apinheiro at igalia.com)