This should ensure that as long as the style does not change the cell
style name is the same after an import export cycle.

Each ScPatternAttr stores a unique ID and we store the ID to name
mapping during import. During export if we find a ScPatternAttr that has
a key that is also stored in the map we write back the style name from
the map.
To avoid name collisions we block the style names from the import for
the export.

The missing piece to make this completely awesome is now to make sure
that styles are sorted by name during export. That way we can reduce the
diff between import and export even more.

Change-Id: Ie4fe2aa00f07efec27ea129e314ac0b6b7e0d8c0
Reviewed-on: https://gerrit.libreoffice.org/29255Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>