Remove menubar from toolbar

- Up until now we placed the Vim menu inside a toolbar. This allowed
  the menu to float detached from the window - however this breaks the
  menu for some external tools, namely dbusmenu access/unity and the
  external menu for Bespin.
- This commit removes our own menubar and uses the mainwindow default
  menubar instead.
* This seems to trigger some issue with shell resizes.
* Issue#101