Include CSS as GResource

Also use explicit namespace for bare functions and constants used in
main.rs.

Cargo.toml

@@ -9,6 +9,10 @@ license = "GPL-3.0-or-later"
 [profile.release]
 lto = true
 
+[build-dependencies]
+# Keep version in sync with GLib as included in GTK4
+glib-build-tools = "0.21.0"
+
 [dependencies]
 async-sqlite = { version = "0.5.4", default-features = false }
 gtk4 = { version = "0.10.3", features = ["v4_20"] }