Multiple overlays displayed at the same time can lock up the game

[agustincordes]

Must investigate this further, but in short: when spots on a node that lie "beneath" an opened overlay -- and which also open an overlay -- are inadvertently clicked, it can cause the overlays to lock-up.

[agustincordes]

Will implement this new attribute for overlays:

Modal - Ignores any click outside the top overlay.
Modeless - The current mode, clicks outside the overlay are transferred to other overlays and the node.

Also a function hideOverlays() that fades out all active overlays -- should be useful for games that make heavy use of overlays.