This is required for giving the Wayland backend working keymap support.

The current gdk model is:

- a display has a single keymap
- the keymap has a keys-changed signal that informs you about mapping changes


In wayland:
- each keyboard device has a keymap
- the keymap object is replaced when the mapping changes


Some widgets, such as GtkEntry, obtain the keymap object once, and connect to state-changed and direction-changed events to keep track of changes. 

Thoughts so far:

- Giving each keyboard device its own keymap is reasonable. We can easily define the display-associated keymap to be the one associated with the core keyboard.

- Replacing the objects is not a great match for our existing code. I suggest to turn the object that is being replaced into an internal keymap->mapping object, and keep the actual keymap object the same for the lifetime of the device.