_use_gtk() created a fresh Database instance on every file dialog
open just to read one setting. Now caches the result at module level
after first check. reset_gtk_cache() clears it when the setting
changes.
Supports Danbooru, Gelbooru, Moebooru, and e621. Features include tag search
with autocomplete, favorites with folders, save-to-library, video playback,
drag-and-drop, multi-select, custom CSS theming, and cross-platform support.
