prevent invalid ods files with loext:fill-character

ODF specifies that extension elements in a document are removed before
the validation. With our old implementation of the number format export
this could result in two number:text elements following each other.

According to the spec a number:text element may not follow another
number:text element.

The number:text element before loext:fill is actually not that
interesting so it makes more sense to make this one an extension
element.

Possible documents showing issues:

fdo45268-1.xlsx
fdo49150-1.xlsx
and many more

Change-Id: I3ad92b8a61a246b3dbc0132d9a2c367488a766d0