Fixed #17

Morph bounds previews are now correctly restored when closing the Parents and Children menus.
Fixed a few issues with m.morph.MenuBar

* m.morph.MenuBar now inherits from m.morph.Flex rather than m.morph.Layout so that it can be adjusted to fill the width of any container
* Menu items in menu bars only open their submenus when left-clicked
Fixed #27

Resize handles now work on m.morph.Text instances.
Removed menubar from simple demo
Fixed m.morph.Inspector instances not being inspectable through the Children menu
Fixed inspector categories showing `undefined` in the value panel
Fixed list items opening when clicked and already selected
Added m.morph.Morph#focusable
Fixed bounds overlays not hiding when developer menus were closed with the Parents or Children menus open