SoundManager pre-loading

- you can now pre load added sounds
(turns every sounds added as class/string to sound object by instanciating or loading them.)

- onAllLoaded signal is dispatched when all external sounds are done loading - even if loaded through playSounds()

- if preLoadSounds() is not called, sounds will be loaded individually on their first call to playSound() which is sometimes better/easier.

- currPlayingSounds renamed to readySounds (sounds that are ready to be played. because this wasn't describing a list of sounds that were actually playing)

- soundIsReady tells if a sound is loaded and ready to play (does not guarantee volume/channel/panning settings exist yet.

- using the sound complete event on newly created channels on which we play a sound, we can know if a sound is playing using soundIsPlaying("sound"). which is turned true when play() is called on the sound and false when the sound complete event is triggered by the channel.