build: Update configure.ac to check GI modules
build: Add PIL and GObject as Python dependencies
Fix convertPixbufToImage

Using GI, subpixbufs share the same pixels as the original pixbufs
but point to the starting pixel given by the subpixbuf's x and y.
This means that, in order to convert the subpixbuf to a Pillow Image,
the right pixels buffer needs to be extracted from the original one.
Fix string encoding issue when setting a box's text
Fix string encoding issue in OdtGenerator
Fix mistakenly commented out frame in boxes area
Fix updating the OCR engines in the BoxEditor

The engines weren't being updated in the BoxEditor after they
were changed from the OcrManager dialog.
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.