OSDMenu improvements

OSDMenu improvements
- new style concat to indicate that images should be put in a region list
- user selected alpha value for subpictures
- position, x, y, timeout, update, alpha values are changeable during runtime
- vlc_osd.h: documented new osdmenu style concat

Known issues:
In osdmenu concat mode there is one issue left where two subpicture regions are displayed on top of each other when cycling through the menu.