Only use the user set page margins for custom paper

Only use the user set page margins for custom paper

When the QPrinter is initalized then it will set up page margins based
on the default paper size. If the paper size is changed to be a custom
one then it should disregard the margins for the default paper size.

If the page margins are set explicitly beforehand then it will use these
page margins.

Change-Id: Ic535c3a80b8b217dbd5eb5f4fb2cbc0ab1354563
Reviewed-by: Titta Heikkala <titta.heikkala@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>