Initialise default styles just once; display an icon for points.

Previously, default styles would be constructed for every placemark.
Now create the default styles just once, then copy as required.

In addition, now display an icon for placemarks with points, but not for
lines or polygons.
Currently, the icon cannot be styled with the Simplestyle properties
"marker-size", "marker-symbol" or "marker-color":
this is yet to be implemented.

Differential Revision: https://phabricator.kde.org/D23533