[ODF] Gradient fill for backgrounds of text documents and spreadsheets

Hi all,

currently LibreOffice writes invalid documents, in case you set the 
background of a text document to a gradient fill. That is 
https://bugs.documentfoundation.org/show_bug.cgi?id=103602

I suggest not to use the loext namespace to solve the problem, but to 
handle it this way:

Add the attribute draw:style-name to the element <style:master-page> and 
let is refer a <style:style> element with style:family="drawing-page". 
This family allows all the needed attributes.

Advantage:
(1) No private namespace needed. The document validates against the 
ODF1.2 schema.
(2) The style family "drawing-page" provides the attribute 
draw:background-size. With its value "full" it would be possible to 
implement the long demanded background fill till page edge without any 
change to ODF1.2.

It might be needed to make some additions to the textual part in the 
ODF1.2 specification in "19.219.27<style:master-page>" to explicitly 
allow this for text documents and spreadsheets too.

What do you think about this approach?

Kind regards
Regina