skins: add some transparency to the default skin

Default skin becomes slightly transparent when moved.
(deactivated by default, needs to be set up in preferences)

share/skins2/default/theme.xml

 <!DOCTYPE Theme PUBLIC "-//VideoLAN//DTD VLC Skins V2.0//EN" "skin.dtd">
-<Theme version="2.0">
+<Theme version="2.0" alpha="255" movealpha="200">
   <ThemeInfo name="subX" author="Martin Poehlmann" email="" webpage="http://www.skinconsortium.com"/>
 
   <!-- Created using the VLC Skin Editor 0.8.0.dev (http://www.videolan.org/vlc/skineditor.html)-->