ENH: Support menus in toolbars (Qt)

Menus can now be added to toolbar_manager, which shows it
as a toolbutton with a drop-down menu. The default action
can be specified by the `action` attribute on the Menu.

![screenshot from 2014-04-21 15 30 16](https://cloud.githubusercontent.com/assets/776111/2753681/e4f31474-c93b-11e3-8d0a-db5790f98eff.png)