Some hot-spots around dynamic_cast of SdrHint, so avoid that by creating
special SfxHintId::ThisIsAnSdrHint

The most common CPU hot-spot was in SvxShape::Notify, the other changes
are just for consistency.

Also remove some dead code in ScShapeChildren, the Notify method was
doing nothing useful.

Change-Id: I696db1fcafc09bb10bf23ac673de292746141491
Reviewed-on: https://gerrit.libreoffice.org/71108
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>