work towards making timeline actions all undoable

app/undo/undostack.cpp

@@ -1,3 +1,12 @@
 #include "undostack.h"
 
-QUndoStack olive::undo_stack;
+OliveUndoStack olive::undo_stack;
+
+void OliveUndoStack::pushIfHasChildren(QUndoCommand *command)
+{
+  if (command->childCount() > 0) {
+    push(command);
+  } else {
+    delete command;
+  }
+}