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).
Vendor microui 2.02 under src/microui/ and use it to draw an in-game
popup menu as an overlay.
modal.c provides generic modal infrastructure: a loop that pumps SDL
events into microui and renders the command list via SDL_Renderer,
hooked into gfx_updateScreen(). menu_sdl.c builds the concrete popup
menu on top of it.
kichikuou