Add new subdirectory for themes

Start with theme 'ub-mannheim' and move our screensaver to that location.
The screensaver is now selected by the 'theme' entry in palma.ini.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
startx: Fix shell used in su command for latest Debian

Debian changed the default shell for www-data, so the old su calls no
longer worked. Fix this by specifying the shell explicitly.

Signed-off-by: Stefan Weil <sw@weilnetz.de>