Load sounds only once and reuse in `SoundManager`

Currently, we have this line: https://github.com/ForNeVeR/overtone/blob/156c60d9336b14c7464b36c5adc1db614980811d/Overtone.Game/UI/Controls.fs#L26

It will load the same sound again and again for each loaded control.

We should reuse the loaded sound somehow.

I propose to introduce a separate class, `SoundManager`, that would manage the loading of sounds — either load everything at once before running the game (for ultimate performance), or load it lazily on the fly. For our amount of data, probably both would work fine.