inputmethod: Allow forcing the display of the input method

Adds support to the new information offered by KWin. Now it can tell us
that the current client doesn't support Wayland input methods.

Therefore, it sets the plasmoid in a new "unsupported" state that will
offer to force an activation when triggered. KWin will then be able to
emulate a keyboard and it all should work to some extent.

Depends on kwin!2907

BUG: 439911