Menu merging

I just added support for menu-merging based on GtkUIManager to the ThunarView. This was the last step in the reordering of the view stuff. Now there are only a few other issues to address before Thunar can be imported into Xfce's SVN repository. Maybe we'll also release an early pre-alpha version as suggested by Botsie.

Not many user-visible changes over the last weeks, but anyways, here's a new screenshot: