ui: Graph legend (issue #308, #540)

* Show graph legend via 'legend' command or '?' button in the lower right corner
* Legend is not interactive: nodes and edges not clickable or moveable
* CXX and Java node types are mixed

src/lib_gui/qt/graphics/QtGraphicsView.h

@@ -67,6 +67,8 @@ private slots:
 
 	void hideZoomLabel();
 
+	void legendClicked();
+
 private:
 	bool moves() const;
 
@@ -104,6 +106,8 @@ private:
 	QtSelfRefreshIconButton* m_zoomInButton;
 	QtSelfRefreshIconButton* m_zoomOutButton;
 
+	QtSelfRefreshIconButton* m_legendButton;
+
 	float m_zoomInButtonSpeed;
 	float m_zoomOutButtonSpeed;
 };