X11 only let's you query the state of mouse button 1 to 5. However, it can
generate ButtonPress and ButtonRelease events for up to 256 mouse buttons.
Instead of asking the server which buttons are pressed, we will now remember
the button state from those ButtonPress and ButtonRelease events. Currently
this let's us keep track of up to 32 mouse buttons.
Signed-off-by: Uli Schlachter <psychon@znc.in>