fdo#69873 FixedText (label) is implemented as no value

And the actual label is put as *body* of the cell.
I'd prefer that the value be put as string-value attribute of the cell,
but since in the report definition the label is as body
(as opposed to as an attribute), it is easier that way.

We could move (actually *copy* for backwards compatibility reasons)
the label to an attribute of the rpt:fixed-content element
(similar to the rpt:formula attribute of rpt:formatted-text)
but it is not obvious this is completely desirable:
Indeed it would keep us from putting anything more complex than a string there
(which we don't do anyway now, but thinking of future extensibility here);
I'll leave the exploration of that idea to the indefinite future.

Change-Id: Ia0f7460718ee35a971117e2f79c0997e17e1095e
Reviewed-on: https://gerrit.libreoffice.org/7865
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>