Update collision detector to handle minimum-distance and repeat-distance seperately.
Update placement_finder to use repeat-distance.
Update group symbolizer to handle minimum-distance and repeat-distance.
The first test shows how an obstacle under the group being displayed
will cause the whole group to not display. Above is an replica of
the same layout using non-grouped symbolizers, which shows that
only the parts which intersect the obstacle will not be drawn.
The second is a line placement test, which just tests placement
of groups on a line with variable spacing.
The third is a test of repeat key in group symbolizer.
The test shows two items with different repeat keys ("foo" and "bar")
and rows of other group symbolizers sensitive to either one. The
reference images show that the "bar"-sensitive rows are displaced
by the "bar" item, but not the "foo" item.
