Fix include guard names
Put the menu bar in a separate class
Put the control box in a separate class
Move typedefs to the top of header files
Remove leading underscore from struct and enum names
main: Rename context struct to gmpv_handle
main: Declare typedef separately
main: Move some code into separate files

- Playlist-related code goes into playlist.c
- MPV-related code goes into mpv.c
- The remaining non-event-handler code goes into common.c
Add copyright year