Menu class enhancements.

The "description" field for menu items is now supported, with the description being displayed as a tooltip on the item. Callables in the "subMenu" field may now have an optional "menu" argument which will be passed the Menu instance, in the same manner as for "command" callables.
"Open Recent" menu item.

Implemented in FileMenu.py, and maintaining a list of recently opened files in ~/gaffer/startup/appName/recentFiles.py. Closes #118.
Merge pull request #129 from johnhaddon/recentFiles

"File/Open Recent" menu item