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