Add marker configuration to coordinates plug-in

[sweco-semara]

Feature Request Description
The coordinates plug-in currently lacks configuration for altering marker icon, offset and scale. If changing marker image for infoclick and coordinate plug-ins, the hardcoded scale of coordinates can give very weird results (i.e. a super tiny coordinates marker, since Hajk's default marker image is huge and scaled down to 15%).

Suggested Solution
As per brief discussion in comments of a historic issue #356, it would make sense to extend the coordinates plug-in with the same admin- and config flexibility that the infoclick plug-in already has.

This would be user-friendly for Hajk administrators, since both plug-ins (infoclick and coordinates) referring to the same marker icon resource would work consistently in Hajk.

Patch
See PR #1499 for an implementation of this feature request.