[AD] Mouse buttons now press and release

["Vincent Penquerc'h" <vincent@xxxxxxxxxx>]

This patch adds MSG_* GUI messages for mouse button presses and
releases. I know this was going to be included in a larger patch
for GUI, but I need it and had implemented workarounds in my game
to "emulate" this in a dirty way. Since I plan to release a demo
of it, I have reimplemented this.
Full backward compatibility is kept (MSG_CLICK).
The only thing I am not really sure about is why there are so
many goto getout; statements; my version will send messages for
all presses and releases detected at a a time. If this is a
problem (but I don't see why there would be), feel free to add
them betweem my SendMessage calls.

-- 
Lyrian

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