ui: Port to libadwaita

This ports the application to libadwaita, enabling us to use the libadwaita stylesheet and
widgets to better implement the Gnome Human Interface Guidelines.

src/ui/mod.rs

@@ -19,3 +19,6 @@
 //! This module contains gtk widgets needed to present the graphical user interface.
 
 pub mod graph;
+
+mod window;
+pub use window::*;