Avoid truncating Vorbis file when saving tags

Rather than writing to the same Vorbis file that tags are being read
from, and thus truncating the file while saving changed tags, write to a
GMemoryOutputStream and then replace the on-disk contents with the
memory stream once it has been completely written.

https://bugzilla.gnome.org/show_bug.cgi?id=721351
Check before flushing ogg stream in vcedit_write
Add British English translation

Created with en_GB.pl: https://wiki.gnome.org/BritishEnglish
User docs: Make page ID the same as file name
Updated Serbian translation
Fix a double free when opening a command-line path

https://retrace.fedoraproject.org/faf/problems/1625976/
Updated Polish translation
Use G_BEGIN_DECLS in more headers

This makes it easier to test the C++ TagLib API for reading MP4 tags.
Use C++ TagLib API instead of C wrapper

Eventually, this will enable handling of cover art and extended tags for
MP4 files.
MP4 TagLib C++ WIP