UI: Disambiguate timeline frame/call arrow markers

Frame and ApiCall arrow markers were both white

Keep ApiCall arrow marker a gradient white. Change Frame arrow marker
to a gradient black.

Don't know if there was any particular reason for the end gradient color
QColor(0xa6, 0xce, 0x39) (history is gone) so used it for both the black
and white gradations.

Testing done with vogleditor

Signed-off-by: Lawrence L Love <lawrencex.l.love@intel.com>