properly reset skin-related settings after changing the current skin (fixes #14595)

The first commit is a cleanup and makes sure that the settings spinners of skin-related settings are always up-to-date. I ran into this issue only once while debugging issue #14595 and wasn't able to reproduce it afterwards.
The second commit is the real fix for http://trac.xbmc.org/ticket/14595 caused by the settings refactor. The problem is/was that the info labels Skin.CurrentTheme and Skin.CurrentColourTheme didn't return the updated values after changing the current skin.