This makes our documentation slightly better (no need to document
default value of Layout.maximumWidth to 100000000)

In addition, the developer can now specify a preferredWidth/Height of 0
and it will actually be respected. (Previously there was no way of
distinguishing between Layout.preferredWidth not set, and set to 0.)

Change-Id: I06ffcf6f801db5847c992e4334cc04e7b7dade4b
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>