plugins: chase audtag changes
More audtag changes.
Move tag_update_stream_metadata() to libaudcore.
skins: Reimplement double size.
skins: Fix some scrolling textbox bugs.
skins: Support skins that don't include an equalizer graph.
Remove mp4ff-based AAC plugin.  Move iTunes cover art support to ffaudio.  Closes: #425.
Use C++ enum syntax.
Add support for dual GTK+Qt build.
qtui: Tweak open files dialog
qtui: Add menu bar (wip)
qtui: plug hook callback leaks