Add shortcuts action to show keyboard shortcuts dialog

Create app.shortcuts action with Ctrl+? shortcut that loads and
displays the shortcuts window from GResources.

src/main.py

@@ -37,6 +37,7 @@ class RosterApplication(Adw.Application):
         self.create_action('quit', lambda *_: self.quit(), ['<control>q'])
         self.create_action('about', self.on_about_action)
         self.create_action('preferences', self.on_preferences_action)
+        self.create_action('shortcuts', self.on_shortcuts_action, ['<control>question'])
 
     def do_activate(self):
         """Called when the application is activated.
@@ -68,6 +69,13 @@ class RosterApplication(Adw.Application):
         """Callback for the app.preferences action."""
         print('app.preferences action activated')
 
+    def on_shortcuts_action(self, widget, _):
+        """Callback for the app.shortcuts action."""
+        builder = Gtk.Builder.new_from_resource('/cz/vesp/roster/shortcuts-dialog.ui')
+        shortcuts_window = builder.get_object('shortcuts_dialog')
+        shortcuts_window.set_transient_for(self.props.active_window)
+        shortcuts_window.present()
+
     def create_action(self, name, callback, shortcuts=None):
         """Add an application action.