fdo#57950 Replace chained append() with operator+

Replaced in the following files:
 sw/source/filter/html/htmldraw.cxx
                      /htmlfly.cxx
                      /htmlforw.cxx

Conflicts:
	sw/source/filter/html/htmldrawreader.cxx
	sw/source/filter/html/htmlfly.cxx

Change-Id: I3884313dd77220ba5d20e6573a6b5dd7a7c77bec
Reviewed-on: https://gerrit.libreoffice.org/8701
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>