Audio: Change how volume works

Across the entire audio system, audio now works in 0-1 terms instead of
0-100 as before. Therefore, volume is now a double instead of an int.
The master volume of the AudioServer changes smoothly through a
FadingProperty, preventing clicks. Finally, volume computations are done
with logarithmic scaling, which is more natural for the human ear.

Note that this could be 4-5 different commits, but as they change each
other's code all the time, it makes no sense to split them up.

Userland/Services/AudioServer/AudioClient.ipc

@@ -4,5 +4,5 @@ endpoint AudioClient
 {
     finished_playing_buffer(i32 buffer_id) =|
     muted_state_changed(bool muted) =|
-    main_mix_volume_changed(i32 volume) =|
+    main_mix_volume_changed(double volume) =|
 }