* skins2: support for custom popup menus, and win32 implementation.

 * skins2: support for custom popup menus, and win32 implementation.
   XML example:
       <PopupMenu id="sample_menu">
           <MenuItem label="Play" action="vlc.play()" />
           <MenuItem label="Pause" action="vlc.pause()" />
           <MenuSeparator />
           <MenuItem label="Show playlist" action="playlist_window.show()" />
           <MenuItem label="Hide playlist" action="playlist_window.hide()" />
           <MenuSeparator />
           <MenuItem label="Quick open file" action="dialogs.fileSimple()" />
       </PopupMenu>

   To call the menu:
       <Button action="sample_menu.show()" ... />

   Known bug: the popup disappears when the user clicks "too fast" on the
   button: the control refreshes itself in an asynchronous way, and the popup
   dislikes it... so click slowly :)