menu: Remove GTK menu backend

Linux and macOS now both build the pure-SDL + microui menu. Remove GTK3
detection, the ENABLE_GTK build branches, and the GTK menu sources.

Decouple gettext NLS from ENABLE_GTK so menu labels stay translatable on all
desktop builds, and update i18n to the new sources (POTFILES.in, .pot, ja.po).

modules/SACT/sprite_tevent.c

@@ -121,7 +121,6 @@ void spev_main() {
 	// デフォルトのコールバックのうち、ここで必要なものだけ
 	// 処理。(VAコマンドcallbackはなし)
 	if (nact->popupmenu_opened) {
-		menu_gtkmainiteration();
 		if (nact->is_quit) sys_exit(0);
         }
 }