Further corrections to drag-end processing.

This time for sure!  (Storing features in the memoized-markers hash
is not usually a particularly happy thing...)
Rebuild the FUSE index with changed features after edits.

To avoid thrashing completely, we do this in a timeout, ten seconds
after things go quiet.
Edit docs materialize as 'readme.edit_mode'.

Some of this text was formerly part of a TODO document.