Treat icons as part of the placemark used for collision detection.

Previously icons were shown in any case, leading to a crowded
appearance when many overlap. Now they are only shown when there is
enough room for them. The old behavior (popular placemarks appear first,
show label only if there is room for it) is kept.

There is a potential follow-up tasks: Instead of a "winner takes it all"
approach, group icons into a "group of placemarks" icon to indicate
there is more to show when zooming in. The current approach allows to
integrate that somewhat easily, however the user interaction needs to be
changed acoordingly.

Please review/test especially for map themes like Atlas. The new
behavior seems fine to me, but I might miss some cases.

CCMAIL: tackat@kde.org