We have a placeholder character in the sw doc model for as-char anchored
frames, so it's possible to have a bookmark before/after the frame or a
non-collapsed bookmark which covers the frame.

The same is not true for at-char anchored frames, where the anchor
points to a doc model position, but there is no placeholder character.
If a bookmark is created covering the start and end of the anchor of the
frame, internally we create a collapsed bookmark which has the same
position as the anchor of the frame. When this doc model is handled by
SwXParagraph::createEnumeration(), first the frame and then the bookmark
is appended to the text portion enumeration, so your bookmark around the
frame is turned into a collapsed bookmark after the frame. (The same
happens when we roundtrip an ODT document representing this doc model.)

Fix the problem by inserting collapsed bookmarks with affected anchor
positions (same position is the anchor for an at-char frame) into the
enumeration in two stages: first the start of them before frames and
then the end of them + other bookmarks. This way UNO API users get their
non-collapsed bookmarks around at-char anchored frames, similar to
as-char ones.

Change-Id: Ic1f173c85d3824afabb5b7ebf3a8594311eb9007
Reviewed-on: https://gerrit.libreoffice.org/46889Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>
(cherry picked from commit 76a4305d)