Mainscreen is now aware of the power ups
Changed initialization of highscore variable
Powerups now disappear after some set time

Time is specified in user settings; for now it is 2 seconds.
Powerups should now also work on touchscreen

Thich will be checked when it is put on the server, so a mobile device
can connect to it and try it out.
Merge branch 'powerupsMainscreen' into development