Add opt-in location maps to Kalendar

This MR contains the addition of maps for both viewing incidence locations and for picking locations for an incidence.

Clicking on the map in the incidence info drawer opens OpenStreetMaps to the incidence's location. Clicking on the map in the incidence editor sets the incidence's location to the place that has been clicked on.

Additionally, writing in the location textfield in the incidence editor now provides suggestions for a location.

Since maps can cause crashing on some systems, they are by default disabled. They can be turned on in the General area of Kalendar's settings.