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)

(cherry picked from commit 93067f37)

-Werror,-Wreturn-type
(cherry picked from commit 0ed64030)

Change-Id: Id6ee30ea467fc24c991547a4c23a9ce14fdd86c7
Reviewed-on: https://gerrit.libreoffice.org/18381Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>