[WIP] Add methods to change the stereomode from our interfaces

This PR adds a method to builtins and JSON-RPC which allows the stereomode to be set/changed by those interfaces. Code compiles and basically works (tested via JSON-RPC) - but I noticed some occasional crashes and the last line in logfile is in this case always "ERROR: CSoftAE::WriteSink - sink blocked- reinit flagged". Threading issue?