library: Port Open File demo to Vala (#783)

* library: Port Open File demo to Vala

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

Author: Diego-Ivan

src/Library/demos/Open File/main.vala

@@ -0,0 +1,37 @@
+#!/usr/bin/env -S vala workbench.vala --pkg libadwaita-1
+
+public void main () {
+  var button_single = (Gtk.Button) workbench.builder.get_object ("button_single");
+  var button_multiple = (Gtk.Button) workbench.builder.get_object ("button_multiple");
+
+  button_single.clicked.connect (open_single.begin);
+  button_multiple.clicked.connect (open_multiple.begin);
+}
+
+private async void open_single () {
+  var default_directory = File.new_for_path (
+    Environment.get_user_special_dir (DOWNLOAD)
+  );
+  var file_dialog = new Gtk.FileDialog () {
+    initial_folder = default_directory
+  };
+
+  try {
+    File file = yield file_dialog.open (workbench.window, null);
+    FileInfo info = file.query_info ("standard::name", NONE, null);
+
+    message (@"Selected file: $(info.get_name ())");
+  } catch (Error e) {
+    critical (e.message);
+  }
+}
+
+private async void open_multiple () {
+  var file_dialog = new Gtk.FileDialog ();
+  try {
+    ListModel files = yield file_dialog.open_multiple (workbench.window, null);
+    message (@"Number of selected files: $(files.get_n_items ())");
+  } catch (Error e) {
+    critical (e.message);
+  }
+}