ZHA store visualization node positions in local storage

[martetassyns]

Hi,

I would like to make it possible to preserve the position of the nodes in a manually adjusted visualization
(physics checkbox is disabled, and user drag and drops the nods to their preferred position).

My proposal is the following:

• when physics checkbox is unchecked:
  if there was no previous position info saved in local storage, save it now
  else load it now
• while the physics checkbox is unchecked,
  update the position info in local storage whenever a drag-end event is received

This has the downside that it is not preserved accross browsers and might remain a hidden feature for many.
However it has the benefit that it's very easy to implement, and will make life easier for at least some people already.

I don't need someone to implement this for me; I think I can manage.
I just want to check if this is an acceptable feature before I put my time in it.

Regards,
Marte