Add minimal AdwApplication with empty window

outlay-gtk/src/main.rs

@@ -1,3 +1,24 @@
+use adw::prelude::*;
+use adw::Application;
+
+const APP_ID: &str = "io.github.outlay";
+
 fn main() {
-    println!("Outlay starting...");
+    let app = Application::builder()
+        .application_id(APP_ID)
+        .build();
+
+    app.connect_activate(build_ui);
+    app.run();
+}
+
+fn build_ui(app: &Application) {
+    let window = adw::ApplicationWindow::builder()
+        .application(app)
+        .title("Outlay")
+        .default_width(900)
+        .default_height(600)
+        .build();
+
+    window.present();
 }