Enable UI scaling in the scenario and implement Linux menus

Patch from @x-qq
Addresses #195

Note: UI scaling for the scenario editor still needs some work,
but it's basically functional.

src/scenedit/scen.menu.hpp

@@ -0,0 +1,42 @@
+// Author: xq, Tuesday 2020-01-28
+
+#ifndef SCEN_MENU_HPP
+#define SCEN_MENU_HPP
+
+// NOTE: this also includes SFML for us
+#include <TGUI/TGUI.hpp>
+#include <vector>
+#include "undo.hpp"
+
+class OpenBoESceneditMenu {
+public:
+
+	OpenBoESceneditMenu(sf::RenderWindow &);
+	
+	bool handle_event(const sf::Event&);
+	void draw();
+	void update_for_mode(short mode);
+	void update_edit_menu(cUndoList const &);
+	
+private:
+	
+	using MenuHierarchy = std::vector<sf::String>;
+	
+	tgui::Gui tgui;
+	sf::RenderWindow& mainPtr;
+	const sf::String internal_menubar_widget_name { "openboe-scenedit-menu" };
+
+	tgui::MenuBar::Ptr build_menubar() const;
+	void add_menu_placeholders(tgui::MenuBar::Ptr&) const;
+	void add_persistent_menu_items(tgui::MenuBar::Ptr&) const;
+	tgui::MenuBar::Ptr get_menubar_ptr() const;
+	bool handle_keypressed_event(const sf::Event&);
+	bool is_control_key_pressed() const;
+	void update_for_mode_0();
+	void update_for_mode_1();
+	void update_for_mode_2();
+	void update_for_mode_3();
+	void update_for_mode_4();
+};
+
+#endif