Move client-side im contexts to gdk

We want to get rid of im modules in the middle of our event propagation. It just doesn't work.

We want to get rid of loadable modules in general.

The rough plan is to introduce a text event that is more or less doing the same as the events in the wayland text protocol, and move client-side im context handling to gdk.

We have canonical 'platform' context implementations everywhere except on X11, where we may still need to continue taking to ibus from the client-side (but from gdk, and without a loadable module).