LibGUI: Explicitly fire a resize event when toggling Calendar mode

Calendar::toggle_mode function performed a call to Widget::resize with
swapped width and height parameters as a quick way to trigger a
resize_event (resize event is generated only when the new rect size is
different from the old one, hence the swapped parameters).

This method does not work when width and height are equal, such as in
the DatePicker widget where width and height are fixed to 200x200. In
such case, calls to Calendar::toggle_mode would not generate a resize
event, leaving tiles uninitialized, therefore the widget was not able
to properly paint the Month view.

This commit replaces Widget::resize call with a manual triggering of
the event.

Userland/Libraries/LibGUI/Calendar.cpp

@@ -80,7 +80,8 @@ void Calendar::toggle_mode()
     set_show_year(!m_show_year);
     set_show_month_and_year(!m_show_month_year);
     update_tiles(this->view_year(), this->view_month());
-    this->resize(this->height(), this->width());
+    ResizeEvent resize_evt(relative_rect().size());
+    event(resize_evt);
     invalidate_layout();
 }