Optional right and middle mouse button support

Added a new compiler conditional 'FLX_MOUSE_ADVANCED' which enables
right and middle click support. Seems to be working fine for flash
(flash player version 11.2+ required) and cpp (windows) - not for html5
though. It probably also doesn't work with recording yet.
Merge pull request #352 from Gama11/dev_mouse

Optional right and middle mouse button support