Port gnomeglade to gtk.Builder, and update file names

This is a simplistic barely-functional port of the gnomeglade Component
class to use gtk.Builder instead of glade.XML. Custom widget creation
is not yet supported, and translation support is broken.

data/ui/*.glade: Automated conversion to .ui

dirdiff.py: Update to follow .ui file name changes

filediff.py: Update to follow .ui file name changes

findbar.py: Update to follow .ui file name changes

gnomeglade.py: Port to UIBuilder usage

meldapp.py: Port About dialog to gtk.Builder usage, remove gtk.glade
            requirement, and update to follow .ui file name changes

preferences.py: Update to follow .ui file name changes

vcview.py: Update to follow .ui file name changes