* skins2/src/theme.cpp: Improved the saving/loading of skins config

 * skins2/src/theme.cpp: Improved the saving/loading of skins config
   - the format of the skins2-config variable has changed (not backwards-compatible)
   - the active layout and its size are now restored
   - the windows and layouts are now identified by their id, instead of their
     order in the XML file; this fixes problems of disappearing windows
     when editing a skin (commenting a Window for example)
   - it should now be impossible to have all windows hidden at the same time
     (but there is no check that the windows are on screen)