Avoid C++ string conversion warnings

Change the type of string literals to const gchar * in
ET_File_Description.
Use C++ TagLib API for MP4 tags

Eventually, this will enable handling of cover art and extended tags for
MP4 files.
Support extended fields for MP4 tags

Support the extended tags that are supported by TagLib, including total
track number, disc number (and total), composer, copyright and encoded
by.

https://bugzilla.gnome.org/show_bug.cgi?id=701506
Add G_DECL boilerplate to picture.h

The new taglib C++ reader needs to use picture.h from C++ source, add
G_BEGIN_DECLS and G_END_DECLS for correct symbol naming.
Implement cover art support for MP4

Adds support for saving cover image into MP4 files, and show the images
tab for them as well.

https://bugzilla.gnome.org/show_bug.cgi?id=701506
Support album artist in MP4 files

Use the atom directly, rather than going through the properties
convenience API.
Use C++ TagLib API for MP4 header information