Miscellaneous C++ conversion.
Fix a GTK warning due to passing an invalid path to gtk_tree_view_set_cursor().
audtag: C++ object system conversion
audtag/core: fix compile error caused by git
audtag/ape: fix public members
audcore: use ssize_t when we want ssize_t, not int64_t
audtag: fix tag module registration
audtag: fix public interface namespacing
audtag/core: hide the TagModule base constructor from public use
Revert recent libaudtag changes as they broke the build in addition to various other problems.
playlist manager: Reduce size of buttons.
audtag: remove tag_set_verbose() as it's completely disconnected from the codebase
audtag: move audtag public functions to `audtag` namespace.

`tag_update_stream_metadata()` is left in global namespace, instead we mark it with a notice to
investigate moving it to the libaudcore library as it's not related to tagging.
audtag: convert builtin tag modules to C++ objects.

unlike the first attempt, we just do the lame thing and call a setup routine to register everything in the right order.
audtag: give APE builtin priority over id3v1.
Merge pull request #17 from kaniini/master

audtag C++ refactoring v2
audtag: Use static initialization.
audtag: Use C++ style enum.
Merge remote-tracking branch 'origin/master' into gtk3

Conflicts:
	src/libaudgui/plugin-prefs.cc
	src/libaudgui/prefs-widget.cc
	src/libaudgui/prefs-window.cc