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/utility/messaging/type/activation/MessageActivateLegend.h

@@ -0,0 +1,23 @@
+#ifndef MESSAGE_ACTIVATE_LEGEND_H
+#define MESSAGE_ACTIVATE_LEGEND_H
+
+#include "utility/messaging/Message.h"
+#include "utility/messaging/type/MessageActivateBase.h"
+
+class MessageActivateLegend
+	: public Message<MessageActivateLegend>
+	, public MessageActivateBase
+{
+public:
+	static const std::string getStaticType()
+	{
+		return "MessageActivateLegend";
+	}
+
+	std::vector<SearchMatch> getSearchMatches() const override
+	{
+		return { SearchMatch::createCommand(SearchMatch::COMMAND_LEGEND) };
+	}
+};
+
+#endif // MESSAGE_ACTIVATE_LEGEND_H