Spiderfy picture markers if there are collisions

As suggested by @RobinP_1 in #46 (closed), the picture markers collisions make it impossible to hover the ones which are behing.

There is a leaflet plugin which implements "spiderfication" like in Google Earth : https://github.com/jawj/OverlappingMarkerSpiderfier-Leaflet

It could be used in GpxPod to be able to see every picture marker when they are too close to each other.