Blender Git Loki

Fix for glitch with menus not reliably setting an active item (D674)

When menus are clamped to the window bounds,
its was possible not to have an active menu-item under the mouse,
Making Ctrl+S,Enter not completely reliable.

Changes needed to support this are:

- menu item is activated on popup menus
(to avoid relying on mouse-over)
- moving mouse away from menu items only de-activates when over a new menu-item.
- Mouse clicks are ignored if they are not directly over the menu item.