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/component/controller/CodeController.cpp

@@ -152,6 +152,11 @@ void CodeController::handleMessage(MessageActivateFullTextSearch* message)
 	showCodeSnippets(getSnippetsForCollection(m_collection), params);
 }
 
+void CodeController::handleMessage(MessageActivateLegend* message)
+{
+	clear();
+}
+
 void CodeController::handleMessage(MessageActivateLocalSymbols* message)
 {
 	CodeView* view = getView();