GUI: try to detect Gnome font preferences as well and cache the result

Thanks to Caspar Schutijser (cschutijser on IRC) for reporting a regression
in commit cff3b86 on his system.
GUI: enable message scrolling via Home and End

Even with a pretty animation.

Signed-off-by: Jan Kundrát <jkt@flaska.net>
GUI: remove whitespace from the output of gconftool-2
GUI: try to detect Xfce and then use the Gnome font preferences