library: Port Avatar demo to Vala (#781)

* library: Port Avatar demo to Vala

* Use `.begin` to call async functions in Avatar Vala demo

Author: Diego-Ivan

src/Library/demos/Avatar/main.vala

@@ -0,0 +1,29 @@
+#!/usr/bin/env -S vala workbench.vala --pkg libadwaita-1
+
+private Adw.Avatar avatar_image;
+
+public void main () {
+  avatar_image = (Adw.Avatar) workbench.builder.get_object ("avatar_image");
+  var button = (Gtk.Button) workbench.builder.get_object ("button");
+  button.clicked.connect (select_image.begin);
+}
+
+private async void select_image () {
+  var filter = new Gtk.FileFilter () {
+    name = "Images"
+  };
+  filter.add_pixbuf_formats ();
+
+  var file_dialog = new Gtk.FileDialog () {
+    title = "Select an Avatar",
+    modal = true,
+    default_filter = filter
+  };
+
+  try {
+    File file = yield file_dialog.open (workbench.window, null);
+    avatar_image.custom_image = Gdk.Texture.from_file (file);
+  } catch (Error e) {
+    critical (e.message);
+  }
+}