Changed the stepping for adjusting property value from default one
to something more sensible for properties where stepping of one didn't
make sense, usually due to very restricted value range.

Also fixed a couple of miscellaneous issues in designer pages.

Change-Id: I0c1c6ebe2561eca3908647f8738539332d0a376c
Fixes: QDS-2287
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>
(cherry picked from commit c50c02e4)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>