support for POIs from external list

[species]

It should be possible to display a predefined list of OSM POIs (e.g. when someone wants to show handpicked restaurants).

The list should be supplied as GeoJSON file. For each entry (in addition to coordinates), a list of tags that have to match OSM objects has to be supplied. (According to the idea of Overpass Permanent IDs).

The plugin should then download the POIs via overpass and a coordinate/tag query for each POI.

In the future, additional tags from the supplied file could be merged in to the popup (e.g. if the user has rated/tagged POIs to his custom scheme).