* all: the skin sliders can now have a background image, which

* all: the skin sliders can now have a background image, which
 depends on the position of the slider. The "Slider" elements now
 have optional attributes "background", which the Id of a bitmap,
 and "nbimages" which is the number of images in the bitmap (it
 is similar to the radialslider). At the moment there is a hack
 to support winamp slider bars: the different images must be
 separated by 3 rows of unused pixels (well, maybe they are
 useful; if someone knows how... ;)
* theme_loader.cpp: convert the file names from zip archives to
  lower case, to avoid mixed case issues in some winamp skins