gtk/main: Do not unset active state on button release w/o implicit grab

gtk/main: Do not unset active state on button release w/o implicit grab

If an active grab gets undone on button press (e.g. closing a menu), we
will receive a button release on the new target even though it didn't handle
the button press, and disable ::active state.

This causes warnings when handling the button release, as it tries to undo
::active state that is not really there.

In order to fix this, check that the pointer focus actually had an implicit
grab at the time of receiving the button release, before trying to unset
the ::active state.