tdf#91920 sw page gutter margin: handle mirrored margins

tdf#91920 sw page gutter margin: handle mirrored margins

- SwPageDesc::Mirror: generate "right gutter margin" from gutter margin
  for mirrored pages, we just lost the gutter margin here previously

- SwBorderAttrs::CalcRight: handle right gutter margin, so gutter
increases the right margin, not the left margin on mirrored pages

- lcl_CalcBorderRect: similar to left and top margins, compensate for
  right margin gutter as well, so borders are independent from the gutter
  margin (Word compat)

(cherry picked from commit ffe7fd5c3f3de474b201fbb1e25b8251cb13574d)

Conflicts:
	editeng/source/items/frmitems.cxx
	include/editeng/lrspitem.hxx
	sw/source/core/layout/paintfrm.cxx

Change-Id: Ie4d3459ab6edcc60b20c2fed08dbf45060ca9828
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/110692
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>