Add possibility to use images as background or handle of slidevalue

* Add tag <resource background="x" handle="y" handlehover="z" />
  to the slidervalue object.
* All images are optional, e.g. you can use just handle or just
  background image or no images whatsoever

Change-Id: Ie0fee92965c6e883c4e532732756008dcc9e385e
Signed-off-by: Vojtech Bocek <vbocek@gmail.com>