[REVIEW-3-6] fix background color export for cond formats

[Markus Mohrhard]

Hey,

[1] correctly exports the background color in conditional formats to
xlsx. This patch also simplifies the exported xml content a bit and
brings it in line with excel's export.

The patch only affects cond format export to xlsx and leaves all other
features untouched.

Regards,
Markus

[1] http://cgit.freedesktop.org/libreoffice/core/commit/?id=232f2513fc79120ab2625db867f49c35838830ef