easier validation

Aspect ration in subtitle means use underlying video aspect ratio.

WARNING: Since this parameter was ignored before probably there are a
number of image subtitle that have set this incorrectly. DVD's seemed
to be a case in point and that'd been fixed. Fixing other subtitles
should just be a matter of setting the i_aspect parameter correctly.

   most other controls. The text is not really resized though (only cropped
   when needed).
   The change in the DTD is backwards-compatible.

* better support for multiple display selection on OS X. (Remembers last selection of screen between vouts)

* modules/misc/xml/*, include/vlc_xml.h: new xml_ReaderUseDTD() api to enable/disable DTD validation.

works with xtag for now due to lack of a DTD

read entire stream. make xml_ReaderValue return pcdata when available. Added
some more debug printfs when XTAG_DEBUG is defined.

 beos/*: use B_KEY_DOWN/B_KEY_UP messages instead of KeyDown()/KeyUp()
   so we can have access to the key code and be more reliable (fixes a
   bunch of hotkeys). Fixed snapshots (missing pf_control)

There are still other deadlocks in wx though

added a pf_destroy to stream_t, changed all various stream_FooDestroy to
stream_Delete

mkv.cpp: moved seek code in classes, improved support for cue-less files, improved cursor behaviour when seeking