Chaos when a new window is opened during a drag operation

[timboudreau]

Given:

• A view which has had draggable() called on it
• which opens a new window in its drag start handler (and will close it on mouse-up)

things start to go very wrong:

• Events are not delivered to the newly created window until the mouse-up over it that should end the drag operation
• The drag operation, as far as the main window and drag-start view is concerned, never ends - any future mouse motion event over it will generate new drag events, but no mouse-up over the view will end the drag operation

It looks like the root problem here is that a distinct AppState is created for each WindowHandle - it isn't really "application state" (though it should be). So the tests of fields of cx.app_state in WindowHandle.event don't get the truth of the application's state, just what one window's handler's have been able to learn about the application's state.

It seems like a fix would start with AppState being shared in some fashion between windows and owned by the Application or ApplicationHandle instead of being created in isolation in WindowHandle.new().