nodeview: implemented copying/pasting nodes

Re-uses XML-based project saving/loading functions to store nodes, their
parameters, and their connections in a text-based format in the clipboard.

app/project/project.cpp

@@ -23,7 +23,7 @@
 #include <QDir>
 #include <QFileInfo>
 
-#include "common/xmlreadloop.h"
+#include "common/xmlutils.h"
 #include "core.h"
 #include "dialog/progress/progress.h"
 #include "window/mainwindow/mainwindow.h"
@@ -137,3 +137,8 @@ ColorManager *Project::color_manager()
 {
   return &color_manager_;
 }
+
+QList<ItemPtr> Project::get_items_of_type(Item::Type type) const
+{
+  return root_.get_children_of_type(type, true);
+}