Instead of preferring the layout size, and only using the panel size
as a fallback, take the maximum of the two. This way styles can provide
a default size that will be respected. Furthermore, replace unnecessary
function calls with declarative property bindings.

Change-Id: Iabc59c694ac8c8302cea72f53c9177462d7f146c
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>