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.
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).
If -debug_dap flag is given, xsystem35 speaks the Debug Adapter Protocol
(https://microsoft.github.io/debug-adapter-protocol/) via stdin/stdout.
This enables xsystem35 to work as a backend of VS Code's Debugger (and
possibly other debuggers that support DAP).
This removes X11 specific code, making SDL2 a hard dependency.
X-based environments are continue to be supported via SDL2.
This also removes joystick_linux.c which was only used if SDL2 was not
available.
kichikuou