Mapping: More user control over the location from where mapping continues.

Mapping:  More user control over the location from where mapping continues.

Mapping call a function for each matching entry.  So far this has
always assumed that the entry stays in the buffer and search can
continue from there.  However, when the mapper function removes the
tree, more control is needed to specify from where the search should
continue.

The action function handed to the mapping function can now set the
variable `org-map-continue-from' to the position from where mapping
should continue.