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/Applications/Piano/Music.h

@@ -30,7 +30,8 @@ constexpr int buffer_size = sample_count * sizeof(Sample);
 
 constexpr double sample_rate = 44100;
 
-constexpr double volume_factor = 1800;
+// Headroom for the synth
+constexpr double volume_factor = 0.1;
 
 enum Switch {
     Off,