Add gnsu-askpass
Replace yad with gnsu-askpass

I'm replacing the yad-based dialog with one I wrote in pygtk. It removes
the yad dependency and has the advantage that pygtk buttons are
translated in many more languages than the yad ones are.
Install gnsu-askpass files
No need to export GNSU_PASSWORD multiple times
Remove fullstop from string

The already translated strings do not have a full stop
Create the pot file from the gnsu-askpass file
Update pot and po files