Replace old item 'potmeter' by new item 'pimage'.

Recent versions of the X11/GTK GUI didn't allow to control a potmeter,
because that didn't seem to make any sense.

In order to get rid of the confusing potmeter that doesn't distinguish
from a hpotmeter and in order to allow the more useful behaviour recent
versions of the X11/GTK GUI have been utilized (and because we're still
supporting item 'potmeter' for reasons of compatibility with old skins),
introduce new item 'pimage' that reuses most of the current potmeter
code.

Additionally, remove remaining code and documentation of 'potmeter'.


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37016 b3059339-0415-0410-9bf9-f77b7e298cf2
Rename SimplePotmeterPutImage() SinglePhasePutImage().

This is a more appropriate name.


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37017 b3059339-0415-0410-9bf9-f77b7e298cf2
Revise skin documentation.

Relocate the explanation on item 'decoration'.


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37018 b3059339-0415-0410-9bf9-f77b7e298cf2