Register Marble app, part & thumbnailer for application/vnd.geo+json

Summary:
Don't be shy, tell the system that Marble has a plugin for handling geojson
:)

Test Plan:
Created a sample file with the example from en.wikipedia.org/wiki/GeoJSON
My shared-mime-info has application/vnd.geo+json already (added in Oct
2015). Installing with the patch and triggering kbuildsycoca5, Marble is
called as handler when clicking on the sample file in Dlphin as well as does
the thumbnailer show a preview (need enabling in Dolphin settings as usual)

Reviewers: rahn, nienhueser

Reviewed By: nienhueser

Projects: Marble

Differential Revision: https://phabricator.kde.org/D1492