Add icons to drag and drop
Move WindowMover to BansheeEditor

end drag doesn't get called unless mouse is released over a window
 - Capture mouse on start drag (SetCapture, and release with ReleaseCapture)
 - End drag should be handled by GUIManager, but if it doesn't process it DragAndDrop manager should check if mouse was released, and
   release capture and activate end drag callback
When I do drop it over a window an exception with layout happens

Add highlight to WindowMover so I can know when I'm mousing over it with a dragged window in hand


Drag and drop manager currently ignores the provided icon, but it should use it as a cursor

Prevent docking if available size is less than 20 pixels, otherwise there might be some weirdness

GUIViewport:
 - MAJOR TODO - GUIViewport updateRenderElementsInternal only gets called when contents change, but viewport should update even if 
    only its offset changes (normally that just marks the mesh as modified, which doesn't result in a call to updateRenderElementsInternal)
	- UPDATE: I don't use GUIViewport anymore

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Other things to remember:
 - Possibly add a way to have hidden widgets in the EditorWidgetContainer (like side-bars that pop on mouse over in Visual Studio)
 - A way to persist window states
 - Also a way to reset all widgets to original locations