Add controls for panning

[Malvoz]

Background: Maps4HTML/MapML-Specification#88 (comment).

• Add (default) controls for panning (e.g. https://kartena.github.io/Leaflet.Pancontrol/)
• Support the nopan keyword in controlslist to disable the controls

[Malvoz]

Side-by-side/grouped controls for panning take up a lot of space in the map viewer. Another consideration would be "border pan" controls (e.g. https://github.com/slara/Leaflet.BorderPan).

See related: Maps4HTML/HTML-Map-Element-UseCases-Requirements#148 (comment).