Allow setting Map.dragging

[davidbrochart]

Fixes #977

It works, but when dragging is set the second time, we get this error in the console:

272.eb0d340….js:1 Error setting state: Cannot convert undefined or null to object
t.set_state	@	272.eb0d340….js:1
(anonymous)	@	272.eb0d340….js:1
Promise.then (async)		
t._handle_comm_msg	@	272.eb0d340….js:1
_handleCommMsg	@	jlab_core.fc632a0f38…32a0f38747f007842:2
_handleMessage	@	jlab_core.fc632a0f38…32a0f38747f007842:2
await in _handleMessage (async)		
(anonymous)	@	jlab_core.fc632a0f38…32a0f38747f007842:2
Promise.then (async)		
_onWSMessage	@	jlab_core.fc632a0f38…32a0f38747f007842:2

Looks like a low-level widget comm message error. Any idea @martinRenou ?

[martinRenou]

Do we really need to rerender the entire map just because dragging has been changed? You could probably use map.dragging.enable(); and map.dragging.disable(); instead?

[davidbrochart]

Indeed!