general: Use adw alias for libadwaita

This will make writing libadwaita-based code a bit
nicer, and make it nicer when we use crates that
libadwaita's API re-exports.

Also clean up the unused imports exposed by
this change.

src/main.rs

@@ -30,7 +30,7 @@ use application::LpApplication;
 
 fn main() {
     gtk::init().expect("Failed to initialize GTK.");
-    libadwaita::init();
+    adw::init();
     pretty_env_logger::init();
 
     setlocale(LocaleCategory::LcAll, "");