ModifiablePath is an Openlayers Class similar to Openlayers.Handler.Path

When placing points with standard Path hander, once a point has been placed,
it's not possible to modify it.

With ModifiablePath, it's possible to drap points of the path to move them.
When a new point is entered, a temporary middle point appears. That point can
be used to split portion of a path. So, new points can be added as needed.

Also, when pressing Shift key, yes enter in delete mode. When in delete mode,
clicking on a point of the path deletes the point. Releasing Shift key leaves
delete mode.

Once path has been finalized (by double clicking), the path cannot be modified
anymore.