added clone functions for nodes

Since nodes are are extremely derivative types, we use a clone/copy function
to produce new types of the derived class easily.

app/widget/timelinewidget/undo/undo.cpp

@@ -25,7 +25,7 @@
 #include <QDebug>
 Block* CreateSplitBlock(Block* block, rational point, QObject* parent = nullptr)
 {
-  Block* copy = block->copy();
+  Block* copy = static_cast<Block*>(block->copy());
   copy->set_length_and_media_in(block->length() - (point - block->in()));
   copy->setParent(parent);