fdo#69282: sw: fix updating of page styles via SwDocShell::_LoadStyles()

SwDoc::CopyPageDesc() only copies master and left SwFrmFmt attributes,
but not first-master and first-left.  They will contain exactly the same
attributes as master and left but they still need to be copied...

(see also: that FIXME in pagedesc.hxx)

(regression from 75084f6c42c27dc95418df9cefed2fddfb26000e)

Change-Id: I3dcc3627708b5d6a477eb7fef76cf6c42c95c004
(cherry picked from commit 5c1cc92ee09f9fcc99077cacd3fc55640f03b7b2)
Reviewed-on: https://gerrit.libreoffice.org/10772
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>