wlt: toolkit: allow widgets to control new size

wlt: toolkit: allow widgets to control new size

This changes the prepare-resize logic to allow widgets full control of the
new size. The initial size starts at 0 and each widget can increase it.
The last widget should be a catch-all that uses all size that is available
and computes a good new size based on the requirements that the previous
widgets provided.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>