Refactor the game event loop and add menus to the Linux build

Patch from @x-qq
Addresses #195

src/game/boe.menu.hpp

@@ -0,0 +1,47 @@
+#ifndef BOE_MENU_HPP
+#define BOE_MENU_HPP
+
+// Author: xq, Saturday 2020-01-25
+
+// NOTE: this also includes SFML for us
+#include <TGUI/TGUI.hpp>
+#include <vector>
+#include "universe.hpp"
+#include "boe.consts.hpp"
+#include "spell.hpp"
+#include "skills_traits.hpp"
+
+class OpenBoEMenu {
+public:
+
+	OpenBoEMenu(sf::RenderWindow&, cUniverse&);
+
+	bool handle_event(const sf::Event&);
+	void draw();
+	void update_for_game_state(eGameMode overall_mode, bool party_in_memory);
+	void update_spell_menus();
+	void update_monsters_menu();
+
+	private:
+
+	using MenuHierarchy = std::vector<sf::String>;
+
+	tgui::Gui tgui;
+	sf::RenderWindow& mainPtr;
+	cUniverse& univ;
+	const sf::String internal_menubar_widget_name { "openboe-menu" };
+	std::vector<unsigned int> spell_menus_connection_ids;
+
+	tgui::MenuBar::Ptr build_menubar() const;
+	bool handle_keypressed_event(const sf::Event&);
+	bool is_control_key_pressed();
+	void add_menu_placeholders(tgui::MenuBar::Ptr&) const;
+	void add_persistent_menu_items(tgui::MenuBar::Ptr&) const;
+	tgui::MenuBar::Ptr get_menubar_ptr() const;
+	void purge_spell_menus(tgui::MenuBar::Ptr&);
+	void update_mage_spells_menu(tgui::MenuBar::Ptr&);
+	void update_priest_spells_menu(tgui::MenuBar::Ptr&);
+	MenuHierarchy menu_hierarchy_from_spell(const cSpell&) const;
+};
+
+#endif