Support customized subtitles.
- font color, size, opacity, edge style,
- background color, opacity,
- subtitle position, align, etc.
- add quick menu for subtitle
- [DH] lib/gdi/font.cpp
- [DH] lib/gdi/font.h
- [DH] lib/gdi/gpixmap.h
- [DH] lib/gdi/grc.cpp
- [DH] lib/gdi/grc.h