timeline/various: implemented block copy/paste

Implemented the ability to copy/paste blocks/clips in the timeline. This did
require some large scale changes and reworking of the copy/paste system
introduced a few commits ago, but should be largely functional now.

app/project/item/folder/folder.cpp

@@ -44,7 +44,7 @@ QIcon Folder::icon()
   return icon::Folder;
 }
 
-void Folder::Load(QXmlStreamReader *reader, QHash<quintptr, StreamPtr> &footage_ptrs, QList<NodeParam::FootageConnection>& footage_connections, const QAtomicInt *cancelled)
+void Folder::Load(QXmlStreamReader *reader, XMLNodeData& xml_node_data, const QAtomicInt *cancelled)
 {
   XMLAttributeLoop(reader, attr) {
     if (cancelled && *cancelled) {
@@ -75,7 +75,7 @@ void Folder::Load(QXmlStreamReader *reader, QHash<quintptr, StreamPtr> &footage_
     }
 
     add_child(child);
-    child->Load(reader, footage_ptrs, footage_connections, cancelled);
+    child->Load(reader, xml_node_data, cancelled);
   }
 }