- whenever SwClients are added or removed, the SolarMutex should be
  locked
- locking the mutex there would be a performance killer
- thus only DBG_TESTSOLARMUTEX() and fixing the fallout on DBG_UTL
  builds
- also fix SwXFlatParagraphIterator
- another missing SolarMutex in Writer UNO
- yet another member of SwXTextPortion needs to be guarded in the dtor
- another missing SolarMutex in Writer UNO: SwXCell

(cherry-picked from:
 ffdc5db2,
 c20930bf,
 e61465d2,
 453cf8a4,
 f019f5d6)

Change-Id: I1c8413cd0b9baa7f38e452e94f33e29bc4cf6717
Reviewed-on: https://gerrit.libreoffice.org/19856
Reviewed-on: https://gerrit.libreoffice.org/20089Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Tested-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>