Make repeating a subtree-restricted export command remember the subtree

Make repeating a subtree-restricted export command remember the subtree

Export commands can be restricted to a specific subtree.  When using a
prefix argument to org-export-dispatch, the restriction did not stick,
i.e. the command would be restricted to whatever note the cursor
happens to be in.  This made it difficult to repeat export of a
subtree while editing it.  This change introduces a new marker and
uses this marker to remember the cursor position of the previous
export command.  So repeating a subtree-restricted command will now
re-export the same subtree, provided you are still in the same
buffer.  To break this connection, just make a new export without
relying on the prefix argument.

* lisp/ox.el (org-export-dispatch-last-position): New variable
  (org-export-dispatch): Save position of cursor at the moment when
  the export command is called.  Restore that position temporarily
  when repeating the previous export command.