region: Use a hand-crafted input sources list

Instead of asking IBus, we want to use an hand-crafted list of input
sources that we are sure that a) is intelligible for the user and b)
lists both an XKB layout and an IBus engine that work together.

For now we just hard-code this list but we might want to move it into
its own file and maybe even to some other module.

This also removes the dependency on IBus.