GUI: prepare for a per-channel oscilloscope?

currently supported systems:
- Amiga
- AY-3-8910
- Dummy System
- OPLL
- SegaPCM

no trigger supported yet!

src/engine/platform/abstract.cpp

@@ -33,6 +33,10 @@ DivMacroInt* DivDispatch::getChanMacroInt(int chan) {
   return NULL;
 }
 
+DivDispatchOscBuffer* DivDispatch::getOscBuffer(int chan) {
+  return NULL;
+}
+
 unsigned char* DivDispatch::getRegisterPool() {
   return NULL;
 }