calendar: Rename "Clear All" button

While it hasn't really cleared everything previously (media notifications),
it does less so now. Update the button label to reflect that.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/262

js/ui/calendar.js

@@ -1108,7 +1108,7 @@ var CalendarMessageList = new Lang.Class({
         box.add_actor(this._scrollView);
 
         this._clearButton = new St.Button({ style_class: 'message-list-clear-button button',
-                                            label: _("Clear All"),
+                                            label: _("Clear"),
                                             can_focus: true });
         this._clearButton.set_x_align(Clutter.ActorAlign.END);
         this._clearButton.connect('clicked', () => {