Add support for streaming sounds

The current soundsystem can only play "static" sounds, that is sounds where the whole sound has been loaded into a memory buffer at once. This works well for small sounds, but as we add more sounds, and larger sounds, we need to look at also implementing support for "streaming" sounds.
A streaming sound is a sound where not all of the sound data is loaded at once. Instead only parts of the sound is in memory, and as the sound is played, parts that have been processed are discarded and new data is loaded. In order for this to work well we however need to make sure that these parts are nicely handed for multiple sounds. If there's only one instance of a certain sound playing at once it's quite easy to handle, but if there's multiple instances of a sound playing we need to make sure that we don't discard parts of the sounds that are in use.