Add search widget to allow place search on map

[prushforth]

[prushforth]

This is first an issue with MapML and MapML-Leaflet-Client. Once resolved there it could be re-opened here, to be managed via the 'controls' attribute and/ or scripted API.

[Malvoz]

Relevant (custom UI) experiment: https://maps4html.org/experiments/api/custom-map-ui/geoname-search-ui.html.