Add new item 'rpotmeter'.

This is the missing counterpart to hpotmeter and vpotmeter
allowing rotary control elements in a GUI skin now.

Based on an idea and a realization by Hans-Dieter Kosch,
hdkosch kabelbw de.

Additionally, update (and revise) documentation.


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37149 b3059339-0415-0410-9bf9-f77b7e298cf2
Add information about updated GUI skins.


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37150 b3059339-0415-0410-9bf9-f77b7e298cf2