[Dev] Menus and Toolbars are now Dynamic Container Blocks

[Donn Denman]

I recently converted Menus and Toolbars into DynamicContainers.  
DynamicContainers are designed to be "mixed in" with ordinary blocks.  
They are rebuilt when the active view changes, allowing 
context-specific menu and toolbar configuration.

 From the user perspective the only change to Menus was that I renamed 
the Menu attribute "menuLocation" to "location", which is an attribute 
that is now shared by all DynamicChildren.  Several changes were made 
to ToolbarItem to make its attributes consistent with MenuItems.

File "MenuBlocks" has been replaced by "DynamicContainerBlocks".  The 
Menu and Toolbar code now lives in DynamicContainerBlocks.

The Menu code has been rewritten, and now works more like normal 
blocks, with instantiateWidget() and synchronizeWidget(), etc.

- Donn Denman