Metadata: rudimentary support for XMP metadataa in MP4-based videos

XMP is a media-metadata standard based on XML which may be used
across a variety of media formats. Some video-processing software
writes XMP data without updating the native metadata fields.
Therefore, we should aim at reading XMP metadata and give priority
of XMP data over native fields.

Pros:
	- Support for *all* common media formats.
Cons:
	- XML (complex, verbose, chaotic).
	- Does not even come close to fulfilling its promise of being
	  well defined (see below).

Implement a simple XMP-parser using libxml2. Connect the XMP-parser to
the existing Quicktime/MP4 parser.

First problem encountered: According to the spec, XMP data supposed
to be put in the 'XMP_' atom. But for example exiftools instead
writes an 'uuid' atom with a special 16-byte uid. Implement both,
more options will probably follow.

Second problem: two versions of recording the creation date were found
  1) The content of a <exif:DateTimeOriginal> tag.
  2) The xmp::CreateDate attribute of a <rdf:Description> tag.

Here too, more versions are expected to surface and will have
to be supported in due course (with an obvious priority problem).

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/xmp_parser.h

@@ -0,0 +1,11 @@
+// Parse XMP blocks using libxml2
+
+#ifndef XMP_PARSER_H
+#define XMP_PARSER_H
+
+#include "units.h" // for timestamp_t
+#include <stddef.h> // for size_t
+
+timestamp_t parse_xmp(const char *data, size_t size); // On failure returns 0.
+
+#endif // XMP_PARSER_H