[GUI] reduced the height of the User color "dot"

It looks a bit clumsy when the tree item is selected because the "dot"
will scratch the edges of the selection.

Change-Id: Ia2c450a0ad403768271e0beaafef30548944f1cc
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1170
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>