sw content controls: introduce a word breaking dummy char at the end

- this way double-clicking on the last word of a content control or the
first word after the content control selects the correct text range,
similar to how the same at the start of content control already worked

- this allows not touching the expand flag in the SwTextContentControl
ctor, which was overwritten by the ODT import (when the content
control was at the end of the paragraph) anyway

- hide this dummy character when accessing the paragraph content via the
UNO API or the text export

- still need to audit a few more places in follow-up commits to maintain
the invariant that content controls have the same dummy char at the
attribute start and end -- somwhat similar to how SwTextInputField
does it with CH_TXT_ATR_INPUTFIELDSTART/END

(cherry picked from commit 1e9416fac497a58be04011cfa0ffce9c74a9e397)

Conflicts:
sw/qa/core/unocore/unocore.cxx

Change-Id: I88763d6db84afedbb865b680f040994c4d6ab7d5
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/134259
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

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