Toolbar changes

[Kai Willadsen <kai willadsen gmail com>]

Meld has long had double toolbars due to an inability to easily merge
items across the different views. Using GtkUIManager fixes this and
lets us get rid of the second view-specific toolbar by merging its
elements into the main toolbar, gaining quite a lot of screen real
estate. For some screenshots, check out:
http://ramshacklecode.googlepages.com/meldtoolbarchanges
(I realised after making these that they don't show the fact that the
in-tab toolbars are gone, but trust me - they are.)

The cost is (currently) a slightly wider toolbar. I have plans to
mitigate this by moving filter toolbar buttons into a drop-down menu
and a few other possibilities. However, drop-down menus from the
toolbar aren't trivial, so I'm planning on committing the current
changes soon, and continuing to work on reducing toolbar width.

Future menu/toolbar UI plans include:
 * make file filters appear in a drop-down menu from the toolbar
 * add text filters to the toolbar, also in a drop-down
 * add file and text filters to the View menus (mostly done)
 * add VC status and directory status filters to the menus (mostly done)
 * make menu tooltips appear in the status bar (done, just need to
decide where in the status bar they should go)

Feedback (and ideas) very welcome.

Kai