Use UTF-8 instead of narrow

Use wide_to_utf8 and utf8_to_wide instead of wide_to_narrow and narrow_to_wide at almost all places.
Only exceptions: test functions for narrow conversion, and chat, which is done in a separate commit.

src/script/lua_api/l_mainmenu.cpp

@@ -1057,7 +1057,7 @@ int ModApiMainMenu::l_get_video_modes(lua_State *L)
 int ModApiMainMenu::l_gettext(lua_State *L)
 {
 	std::wstring wtext = wstrgettext((std::string) luaL_checkstring(L, 1));
-	lua_pushstring(L, wide_to_narrow(wtext).c_str());
+	lua_pushstring(L, wide_to_utf8(wtext).c_str());
 
 	return 1;
 }