refactor the UI of preferences window
prefs window is already bloated. a solution is to move to this style:
http://www.workrave.org/screenshots/windows/gfx/preferences.gif
where on the left we keep the tab labels and on the right we do tab labels the frames we have now MAKING SURE that the height of the window is at max 60% of the current height which is SCARY and doesn't look well on small resolutions (f.e. 800x600)
comments welcome