Optional right and middle mouse button support

Credits go to [this forum thread](http://forums.flixel.org/index.php?topic=6459.0).

I 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 (I guess that's rarely used feature anyway? Not sure, but it can be added later).

The new functions for the two new mouse buttons required a lot of copy-pasting. Perhaps there are nicer ways of doing this, not sure. Either way, seems to be working totally fine for now.