groovectl: minor fix
grooved-mmkeys: move mmkeys support to separate daemon
CMakeLists.txt: clean-up

This reorganizes the CMakeLists.txt file a bit to make it more manageable, and
it also makes X11 support optional.
player: remove ReplayGain support

To be replace with proper support for filters.
player: add proper support for filters
grooved: properly document configuration values
player: support using an user-defined audio output
CMakeLists.txt: add systemd service file for grooved-mmkeys as well
grooved-notify: add desktop notifications daemon
player: emit track changed signal on metadata update too