The new MeldStatusBar subclasses gtk.Statusbar, mucking with internal
packing and the like to provide better behaviour for Meld. The
progressbar has been changed to use pulse mode exclusively (we never
used anything else) and progress text is now shown in the progress bar.
The space on the right of the statusbar is now an 'information area'
that individual pages fill themselves.

data/ui/meldapp.ui: Remove old Statusbars, and create our custom one
meld/dirdiff.py: Adapt to new API
meld/filediff.py: Adapt to new API
meld/ui/statusbar.py: New file for MeldStatusBar class
meld/ui/gladesupport.py: Support UIBuilder construction
meld/meldapp.py: Absorb some on_idle logic into the new status bar
meld/melddoc.py: Remove the 'status-changed' signal and provide a
                 harness for individual page status areas