ui: load project via MenuBar

This change adds a MenuBar to QtMainWindow and adds an action for opening a different project settings XML file. The
message MessageLoadProject is thereon used to load a new project in the Application and clearing the Storage.

src/lib/data/graph/Graph.cpp

@@ -223,6 +223,12 @@ void Graph::forEachToken(std::function<void(Token*)> func) const
 	forEachEdge(func);
 }
 
+void Graph::clear()
+{
+	m_edges.clear();
+	m_nodes.clear();
+}
+
 const std::vector<std::shared_ptr<Node> >& Graph::getNodes() const
 {
 	return m_nodes;