Add locked and unlocked modes

[積丹尼 Dan Jacobson]

Description

Vespucci has locked and unlocked modes.

In locked mode there is no fear of accidentally changing the positions of any data. (Which might accidentally end up in a changeset one later uploads to OSM.)

Currently the only mode in iD is unlocked mode. Perhaps add a little padlock icon at the top of the map, straight out of Vespucci, that users can use to toggle locked mode on and off.

In fact in locked mode iD could make certain assumptions that might make panning the map faster, e.g., no need to set up internal tables (?) for items, instead just render them.

Yes, turning off the data layer (ALT+W) protects the data from accidental mouse drags etc., but also doesn't allow one to see the data.