TODO:
 - Callback on tree item select
 - when update() deletes elements it won't remove them from mAutoExpandedElements or mMouseOverDragElement.
 - When I select elements in tree view via arrow keys, make sure to automatically scroll the scroll-area so that element becomes visible
 - Context menu with rename/copy/paste/duplicate
 - Delete with Undo/Redo support
 - Auto scroll
    - Sliding over top or bottom of the tree view while dragging an element will search GUIElement parents to find a ScrollArea. If it finds one it will attempt to scroll up or down.
 - Support for icons next to tree elements
 - Clicking on an already selectecd element starts rename 
   - Will likely need some kind of check to ignore double-clicks?

MenuBar problems:
 - Clicking the top menu item closes and immediately opens the menu again (it should only close it)
   - Something similar probably also happens with ListBox

 Other:
 - "Ping" effect
 - Copy/Paste/Duplicate
   - This is more of a problem with actual copying of SceneObjects (It's not implemented). I should be able to serialize, and then de-serialize with a new parent as a form of copy.

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LOW PRIORITY:

Shortcut plan:
Replace all GUICommandEventType with two things:
 - Events like CursorMoveUp should just be received a keys + modifiers (shift/ctrl/alt) and the individual element can then decide what to do with them and how to interpret them.
 - Events like Rename, Undo, Redo, Copy, Paste, etc. should be globally set shortcut keys
    - Shortcut keys are globally registered on program start in ShortcutManager
    - Objects can then register callbacks for certain shortcut using mechanism like: addCallback("Undo", onUndo).
     - They should also be able to unregister those callbacks.
    - GUIElement can choose to listen to certain shortcut keys
        - getUsedShortcut is a overridable method that returns a list of shortcuts and callbacks
        - When element comes into focus or loses focus GUIManager will register/unregister those shortcuts with ShortcutManager
          - I should probably prevent one shortcut from possibly triggering multiple callbacks
    - Whenever some key combination or a single key is pressed all elements in focus get send the shortcut key (if that shortcut exists)