Rewrite import and export of custom dashes in ooxml filter (fix)

The import mechanism of custom-dash (a:custDash) was wrong, and imported
wrong values, which causes that if you would import-export-import-export -
you would get inflated values, which might cause a corruption.

The attributes for custom-dash nodes (a:ds) are of type 'PositivePercentage'.
Office will read percentages formatted with a trailing percent sign or
formatted as 1000th of a percent without a trailing percent sign, but only
write percentages as 1000th's of a percent without a trailing percent sign.

During import - LO did not check if it was in '%' format or in
'1000th of a percent' format. So that was fixed. Also - when exporting -
it always exports now in '1000th of a percent' format.

Change-Id: I6bd74df26951974f85173227c832386c70034afb
Reviewed-on: https://gerrit.libreoffice.org/9681
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>