KDE fullscreen fix

KDE 4 apparently makes fullscreen windows virtually higher than the
screen if pqiv is started in fullscreen mode. This led to wrong image
positioning in fullscreen. Fixed by always using the screen size in
fullscreen.
Fixed --hide-info-box

The option was doubled. This partly fixes issue #4.
Fixed no-scaling mode

My implementation sometimes ignored the no-scaling setting, because some
window managers ignore requests for window sizes larger that the screen
size. After this commit, in no-scaling mode, images are always displayed
at 100% if no-scaling is enabled, regarless of the window size. I also
disabled in-window-centering in this mode, because this led to funny
displayements at least in xfwm.

Related to issue #4