#4 More mouse buttons

[Taras Kostiak]

Add that client could use more mouse buttons(5 is max for now). Would be
nice to use side buttons(especially on New Dark Hell, or to connect/detach
ball).

Same feature request in BloodsPilot project
https://sourceforge.net/tracker/index.php?func=detail&aid=2055807&group_id=147895&atid=769892