correct nmsbd font selection + optimizing stuff

if nmsbd.ttf not available in enigma2/fonts folder, do not show in font
selection.
At skin conf save, make this skin as default.
user must generate skin at first install to get it working

no more skin.xml file in skin folder.
user must generate skin at first installation in order to get the skin
working.
this is a security added if for example nmsbd font isn't selectable