The element layouting must always be done without a scrollbar, so
it can be decided, if a scrollbar is needed. If it's needed the
first paint will update the scrollbar accordingly.

This also sets the scrollbar's page size to the control height,
so clicking in an empty area will correctly scroll the widget not
just a single pixel.

Change-Id: Ib9a0eb1952bc0355f683cd8d116c61f9c9d462c4
Reviewed-on: https://gerrit.libreoffice.org/72315
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>