(fixes problem with most-but-not-all filenames containing a colon)
- Only allow letters as Windows drives

- 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.

Less verbose.

THANKS: Proper credits for arabic l10n

Set aout_buffer_t.b_discontinuity in mpeg audio packetizer.
Silent first 3 frames on discontinuity in mad decoder.
(close #590)

field to "attachment://NAME" where NAME is an attachment filename
(returned by DEMUX_GET_ATTACHMENTS)
- Lock input_item_t every time we access p_meta fields (there is still a problem
as playlist code does not always do so...).
- Do not preparse item we have already played.
- Wait that the current playing item is meta parsed before fetching its cover.