sw: proper fallback for first page in InsertNewPage()

For left/right share, the fallback is easy: the left page always falls
back to the right page, if there is a fallback. In case of first page,
the situation is more complicated, as there are multiple fallbacks
(left/right).

Since commit 9ff68a2, first page fallback was decided during registering
part of the SwPageDesc to SwPageFrm, which is bad, as without a document
export/import, a changed IsFirstShared() didn't have any effect: the
LeftFmt/RightFmt was in use, so FirstFmt had no SwClients -> nobody was
notified when that setting changed. This commit changes first pages to
always use FirstFmt, and (if a fallback is needed) sets FirstFmt to
point to LeftFmt/RightFmt as necessary.

Change-Id: I4654baf293d72070fcf7e9eea408a43a31079e60
diff --git a/sw/source/core/layout/frmtool.cxx b/sw/source/core/layout/frmtool.cxx
index 1f190aa..815ffcd 100644
--- a/sw/source/core/layout/frmtool.cxx
+++ b/sw/source/core/layout/frmtool.cxx
@@ -2742,10 +2742,28 @@ SwPageFrm * InsertNewPage( SwPageDesc &rDesc, SwFrm *pUpper,
    SwPageFrm *pRet;
    SwDoc *pDoc = ((SwLayoutFrm*)pUpper)->GetFmt()->GetDoc();
    SwFrmFmt *pFmt = 0;
    // rDesc can't know if the first page will be 'left' or 'right', so if
    // first is shared, let's ignore first here.
    if (bFirst && !rDesc.IsFirstShared())
    if (bFirst)
    {
        if (rDesc.IsFirstShared())
        {
            // We need to fallback to left or right page format, decide it now.
            if (bOdd)
            {
                rDesc.GetFirst().SetFmtAttr( rDesc.GetMaster().GetHeader() );
                rDesc.GetFirst().SetFmtAttr( rDesc.GetMaster().GetFooter() );
            }
            else
            {
                rDesc.GetFirst().SetFmtAttr( rDesc.GetLeft().GetHeader() );
                rDesc.GetFirst().SetFmtAttr( rDesc.GetLeft().GetFooter() );
            }
        }
        pFmt = rDesc.GetFirstFmt();
        if (!pFmt)
        {
            pFmt = bOdd ? rDesc.GetRightFmt() : rDesc.GetLeftFmt();
        }
    }
    else
        pFmt = bOdd ? rDesc.GetRightFmt() : rDesc.GetLeftFmt();
    //Wenn ich kein FrmFmt fuer die Seite gefunden habe, muss ich eben