The dialog/sidebar should not actually create styles that don't exist yet,
because it messes up Undo and the (unused) styles are then unnecessarily
exported to documents.

Due to Writer's ... unusual SwDocStyleSheet class this is a bit tricky.

Add a new function GetItemSetForPreview() and use it from the style preview
code.

The implementation does not use FillPhysical so will temporarily create and
then delete any non-existing styles.

Skip page and numbering styles for now since they don't have a useful preview.

(regression from ca953076)

Change-Id: Id6ee30ea467fc24c991547a4c23a9ce14fdd86c7