Implement preview music properly as a new optional tag in the ssc file. Changed ScreenManager to emit an error instead of asserting if a screen attempts to use SetNewScreen inside an InitCommand. Changed decorations layer to be a proper child instead of stripping away the ActorFrame.

src/NotesLoaderSSC.cpp

@@ -426,6 +426,11 @@ bool SSCLoader::LoadFromSimfile( const RString &sPath, Song &out, bool bFromCach
 					out.m_sMusicFile = sParams[1];
 				}
 
+				else if(sValueName == "PREVIEW")
+				{
+					out.m_PreviewFile= sParams[1];
+				}
+
 				else if( sValueName=="INSTRUMENTTRACK" )
 				{
 					SMLoader::ProcessInstrumentTracks( out, sParams[1] );