Startcenter: when screen is small make doc thumbails smaller

(cherry picked from commit 42c1463b8914cf51dd778ff4b22c56e57a935ccf)

Change-Id: If4fbb66992318221a687710afa77fc98e4f111c3
Reviewed-on: https://gerrit.libreoffice.org/7331
Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>