clutter/backend: Allow unsetting input method
This is needed by GNOME Shell to remove itself as a input method implementation during its shutdown sequence. We can't do it ourself later because at mutters own shutdowns equence, the GNOME Shell Javascript context has by that time already been teared down.
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Technically an ABI change, but without GNOME Shell would warn and not perform the method call.
Required for gnome-shell!2722 (merged)
Edited by Robert Mader