[REVIEW 3-5] fdo#48932 speed up scrolling/typing in huge writer documents

[Caolán McNamara]

fdo#48932 is a large 7000 page document which reacts very slowly on text
input and scrolling around. It appears to be mostly managing the
header/footer and page break indicators which makes it sluggish

b63766b3dd453a82f59db505c736d861f662cc0f make it fast again, and seem to
have no side efforts. Though I'd like to have someone else verify that.

Attached is a backport to 3-5 to apply cleanly.

C.