ibusManager: Close candidate popup upon engine change
The IBusCandidatePopup can get 'stuck' in an active, visible state if
it is on-screen at the point when the input method is changed to a method
that doesn't use such popup (e.g. regular English).
Force the candidate popup to close when the engine is changed.
It will reappear (via regular lookup table update signal) if/when it is
next required.