[fleche] Auto-check documents on scroll when in lazy mode.

[ejgallego]

We introduce a new coq/viewRange notification, from client to server, than hints the scheduler about the visible area of the document.

Combined with the new lazy checking mode, this provides checking on scroll, a feature inspired from Isabelle IDE.

TODO:

• [x] option to disable it (in the client)
• [x] user manual