inputmethod: Allow forcing the display of the input method

This makes it possible to show the input method when using a client that
doesn't support Wayland input methods (e.g. Xwayland).

This adds 2 dbus API entry points:
- activeClientSupported property: That tells us if the current client
doesn't support text-input-v*
- forceActivate method: forces the activation of the input method. This
configures the input method in a state that we can use to forward the
input method's keystrokes to the client, emulating a hardware keyboard.

BUG: 4399911