Make GroupBox size respect anchor margins and update Gallery

We now implicitly add the margins to GroupBox so that you
can easily add them to a layout.

This also gets rid of the silly Sidebar example and adds a much
more useful Layout tab for effective testing.

Change-Id: I6026c32b2a198062d9b11a67245df0529aa61f8a
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>