Start an incremental search for what the selection holds, from where the
selection is, and step to the next resp. previous match at once -- so
the matches that are already lit up (see terminal_image->selection)
are the ones this walks. From there it is the ordinary search below, ^S
and ^R included. \C-s and \C-r
are bound to these.
Fails when there is no such selection, and a binding that fails hands
the key on: without one, ^S and ^R mean to
whatever is reading from the terminal what they always meant, which for
the line editor is its own history search.
^G gives back the view and the selection the
search started from, so a search one did not mean to start costs
nothing.
