The output of a command that has finished can be taken off the
display with terminal_block->fold,
leaving the command that produced it with a triangle in the left margin
and a tally of what is hidden. A click on the triangle toggles it;
unlike a hyperlink it needs no modifier, because the marker belongs to
the terminal rather than to the client.
Folding is a property of the view and not of the text. terminal_image<-contents,
terminal_image<-find, terminal_image<-selected
and terminal_image->select_all
keep seeing what a fold hides; only what is painted, the rows a click
maps to and the scrollbar leave it out. terminal_image->scroll_to
opens a fold it lands in, as does an incremental search whose hit is
inside one.
Closing a fold takes rows out of the window, so a window that was
showing the end of the buffer is pulled back to go on showing it. One
the user has scrolled away from stays where they put it.
terminal_image<-fold_at
says whether a position is on a marker, which is what lets a popup on
the marker be about the one command it stands beside rather than about
the terminal. Class prolog_terminal uses it for terminal_image->block_popup.
