Viewer toolbars

This pull request implements a simple framework for displaying semi-transparent toolbars overlaid in the Viewer. The GLWidget provides the ability to overlay Widgets and the Viewer produces toolbars to control Views via plugs stored on the Views themselves.

The only toolbar content implemented in this pull request is a simple drop down menu for controlling display style in the 3d viewer - additional controls for the 3d and 2d viewers will be implemented in other pull requests.