Pause and unpause music

[LeeSpork]

After pausing and then unpausing the game, the background music will now resume playing from where it left off, matching original behaviour.

Closes #3038

[LeeSpork]

It can be discussed in future issues whether other audio should also use the channel pausing feature implemented here. (Though if so: may want to check all uses of Channel::isPlaying() to prevent any potential bugs – it perhaps might make sense to just entirely replace that method with a isPlayingOrPaused method?)