calendar: Add a11y label to the "Clear" button

The button that clear notifications is labeled as
"Clear". When using a screen reader, this label
doesn't provide enough context about the action
that will happen.

Fix that by adding an a11y label to this button.

Closes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7398

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3192>

js/ui/calendar.js

@@ -967,6 +967,7 @@ class CalendarMessageList extends St.Widget {
             can_focus: true,
             x_expand: true,
             x_align: Clutter.ActorAlign.END,
+            accessible_name: C_('action', 'Clear all notifications'),
         });
         this._clearButton.connect('clicked', () => {
             this._sectionList.get_children().forEach(s => s.clear());