Related: fdo#78151 force outliners in master view to be read-only text

because these contain place-holder text and it doesn't make sense
to be able to edit them (unless some-day we support custom place-holder
prompts)

because they are now read-only add a pair of "show next level" and "hide
last level" features so on right clicking the last outline paragraph
you can make the next one visible, or make the last one invisible

Change-Id: Iea24d810f298156cfe2f32aa53d0515da45e08eb