Dynamic Sound Types

Looking to address issues raised in #521, this allows a Sound Type to be defined via a StringHash or the old enum style and preserves loading the old enum style (but will not serialize it again in the future if the scene is exported).

Editor support added for managed Audio Master Gains and for user defined ones can be dragged onto a line edit to set the underlying string hash value into that field.

By no means a sure thing, only one way of doing this so feedback more than welcome.