This change improves import performance of Excel documents based on
the following changes.

1) Pool styles only once per call.

This removes SfxItemPoolCache instantiation and destruction which
is quite expensive.  This alone cuts import almost by half with
large documents with lots of styles applied.

2) Skip removal of direct formats when setting style.

The old code was iterating through all attribute regions in order
to try to remove overlapping format attributes when applying a
style.  But that's strictly not necessary (and bad for performance)
during import.  This also removes the need to re-apply the direct
formats right afterward.

3) Avoid creating a duplicate ScPatternAttr instance per XF.

There is simply no need to create a clone of the style only to put
it into the styles pool and get discarded.  This saves additional
few seconds.

Basically everything except pItemInfos is under pImp.  This will
make it easier for future refactoring in the implementation area.

The visibility configure test failed to clean up visibility.cxx.
This patch fixes that.

 * observed on gentoo tinderbox

the SC_DLLPUBLIC changes in document.hxx are only necessary until we can
link statically against sc

(related to i#95158, but not quite)