Translate the menu/dialog strings that were left untranslated. Add a CI
step that fails when po/xsystem35.pot is out of sync with the sources.
Also remove unused autotools gettext files.
When a game defines no VolumeValancer[], fall back to two generic
channels so BGM and sound effects can be adjusted independently.
Volume controls are now always available, so drop the conditional menu
handling.
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).
kichikuou