ADDED: minor mode for automatic whitespace insertion

ADDED: minor mode for automatic whitespace insertion

* sweep.pl (sweep_context_callable/2): only consider the context to be
callable if a neck is present up the tree, i.e. not in the head.
* sweeprolog.el (sweeprolog-context-callable-p): new function,
extracted from (sweeprolog-predicate-completion-at-point)
(sweeprolog-electric-layout-post-self-insert-function): new function.
(sweeprolog-electric-layout-mode): new minor mode.