Implement music attenuation on Linux

This is implemented by listening for event changes from MPRIS2 clients
over D-Bus. Unfortunately there is no good API for this nor a good way
to identify current player as there is on Windows, and as such we have
to track and queue them manually just to get a seemingly good enough
result. This also gives us the issue of needing to guess and manually
prioritise which player to use when we first start, as we cannot
determine when something before us started.

playerctld is preferred to be read on systems that have it available,
however that has the behaviour of setting its status to the most recent
change. Those who use this and have media key scripts use it would
likely be expecting such behaviour though.

(All my editors were eager to fix inconsistent CRLF's in the CMakeLists,
sorry)

Author: dakrk