adding horizontal scroll bar to list boxes. Refs: #90.
As proposed by Mathieu Westerweele in CPC 1.7 rc6 for SelectionBox.
In addition:
1. additional width of 5 pixels changed to Ports.point * 3 because then it is device independent.
2. the same changes are also applied to ListBox if not in drop-down mode.

Signed-off-by: Josef Templ <josef.templ@gmail.com>