Refactored SourceImagesSelectorIconView

Now it is called PagesIconView, was separated into its own file
and was ported to GI.
Fix PIL importation
Port customWidgets.py to GI
Rename SelectableBoxesArea and move it to its own file

Now it is called BoxesArea.
Also update the import of the old name accordingly.
Port dataHolder.py to GI
Remove unused import from studioBuilder.py
Port studioBuilder.py to GI
Port lib.py to GI
Port asyncworker to GI
Port graphics.py to GI
Remove unused class
Port widgetModeler to GI
Port widgetPresenter to GI
Update copyright of some files
build: Add new Python files to Makefile.am
Add new files to POTFILES.in
Use the last visited dir when adding a new image

This is only true in the same session but in the future, this should
be added to the configuration so it persists between different
sessions.
Fix error when exporting a PDF with empty text areas
Port pygtk syntax in PDF exportation dialog to GI
Fix PDF output options in ocrfeeder-cli