ibusManager: Make more resilient to completion/engine independent changes

ibusManager: Make more resilient to completion/engine independent changes

Besides user interaction, there's two users of IBusManager.setEngine():

- The code that toggles all IBus engines off on entries with PASSWORD
  purpose.
- The code that toggles completion support on OSK presence.

These are currently pretty oblivious to each other. Make this
interaction more resilient by making all external IBusManager changes
more cautious about directly changing the engine, and revoke properly
the completion mode if it needs be (e.g. changing to a non-XKB engine).

But another notable change is that ibus-typing-booster is now preferred
always, over PASSWORD purpose hints. This is done to avoid possible
doubled attempts to change the current engine (and ensuing IBusInputContext
confusion).

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2512>