Change Voice.Volume to be a persistent volume for the Voice itself. Fix #2976

[CodeLeopard]

Voice.Volume is now independent of the currently playing Note's volume.
This is done by adding a private variable to hold the volume as it was specified by the sound or note being played, leaving Voice.Volume to be the volume for the Voice itself.
Both volume values are multiplied together to get the final volume for the sound.

Fixes #2976

Example script:

Local v0 to GetVoice(0).
Set v0:loop to False.

Local softNote is Note(440, 1, 1, 0.5).
Local loudNote is Note(440, 1, 1, 1).

Local Function PlayNotes
{
	Print("Playing soft note...").
	v0:Play(softNote).
	wait softNote:duration.
	Print("Voice volume is now: " + v0:Volume).

	Print("Playing loud note...").
	v0:Play(loudNote).
	wait loudNote:duration.
	Print("Voice volume is now: " + v0:Volume).
}

Until false {
	Set v0:volume to 1.
	Print("Set Voice:Volume to: " + v0:Volume).

	PlayNotes().

	set v0:volume to 0.5.
	Print("Set Voice:Volume to: " + v0:Volume).

	PlayNotes().

	wait 0.5.
}