sc: fix locking in ScXMLConsolidationContext

Essentially revert commit 4fa05ecc089a027f243e87f76cc9bcd1f70447e4,
plus move the unlock to the dtor for more RAII-ness.

Change-Id: Ie5a9cc183626e3f5b005348606bffa69c4be4f7f
fdo#79269: fix ODF import of style:footer-first

The implementation of SwXStyle's FirstIsShared property is busted, and
that causes xmloff to write the footer-first content into the master
footer.

Change-Id: I520a4929d9d7313da65bcdcf4094f8244382377d
fdo#79269: sw: more cleanup of SwXStyle's FirstIsShared property

Some of the checks for it can never be true given the preceding list of
WhichIds, but there is one other case that needs the special handling.

Change-Id: Iaf396960d064d0c9f2a950c2d02db1654a669d45