Make ooxml-handles work in odp

[Regina Henschel]

Hi Miklos,

Miklos Vajna schrieb am 06-Aug-19 um 09:26:
> Hi Regina,
[..]
> 1) If you want to know which import filter was used to create the doc
> model, you can get that info from SfxBaseModel::getArgs(). One of the
> property values returned by that function has the info.

Thank you for the hint. I have seen the property in the Basic IDE. 
However, I have refrained from the idea. On the one hand 12 comparisons 
would be necessary, namely 4 modules x 3 document types (normal, 
template, flat). That's quite a lot. On the other hand it became clear 
to me that it doesn't work in principle.

You can also insert "ooxml-foo" shapes from the Gallery into a pptx 
document. Then the shape would not have the required attributes and they 
would have to be added although a pptx document was opened. Therefore 
you have to inspect the handles in any case.

The solution with method createItemIfMissing() seems to me to be clearer 
and safer than to test all combinations in a pre-test and treat them 
individually. So that is in Gerrit now.
https://gerrit.libreoffice.org/#/c/76887/

Please have a look at it. If there are no concerns, I'll commit it.

Kind regards
Regina