Clocking: Make insertion of the CLOCK drawer safer

Clocking: Make insertion of the CLOCK drawer safer

Daniel Clemente noticed that the clock drawer, when it is created for
the second clock entry, will swallow a plain list item after it,
because it thinks it might be a clock note.

This commit makes this function check for indentation.  If the plain
list item is indented less than the last clock line, it will be
assumed that it does not belong to the clocking line, and the drawer
will not swallow the item.