Add GTK wallpaper library

Scan ~/Pictures/Wallpapers and /usr/share/backgrounds/bos (configurable)
and open a bread-theme GTK picker via `breadpaper library` (alias browse).
Clicking a thumbnail runs the existing set path. listen honors
bread.command.paper.library by spawning that picker.

bakery.toml

@@ -1,10 +1,11 @@
 name = "breadpaper"
 description = "Wallpaper manager for the bread desktop — sets awww wallpaper, generates pywal palette, reloads bread themes"
 binaries = ["breadpaper"]
-system_deps = ["awww", "python-pywal"]
+system_deps = ["awww", "python-pywal", "gtk4"]
 optional_system_deps = []
 bread_deps = ["bread-theme"]
 license_file = "LICENSE"
+desktop_file = "breadpaper.desktop"
 
 [install]
 post_install = []