ApplicationWindow has no default size

ApplicationWindow should not set a default window size because
every platform already does that, and normally an application would
set its initial width and height to the layout's minimums anyway.

Change-Id: I42ab1b6ecc71a38961f44a67f1c819a6a2037171
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>