Remove popup menu in the pages area

This removal is in order to prevent an error which seems to be
caused by some bug in the GI. It should be better investigated in
the future.
Warn when no OCR engines are found on startup
Warn when there are no OCR engines on recognition

(when recognizing a document/page/area)
Update the box editor's OCR controls sensitiveness

The sensitiveness is updated according to the existence of OCR
engines so the user is not able to perform the OCR recognition
when there are no engines configured.
Port engines migration dialog to GI