More SoundGUI changes:

- manage pan and volume from GUI, directly effects sound
- added self.channel to better manage playing sound in different circumstances
- kill sound and let go of file on close window
- play and stop buttons alternate, if sound is playing, stop is enabled, else play is enabled
- changed PanSlider range to -50,50 and volume range to 0, 100, easier to calculate other values from
- update GUI Text for Pan, Volume based on existing value