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.

Closes: #6717 (closed)