feat: Export as .odp (Open Document Presentation)

[kamazeuci]

Export as .odp instead (or in addition) of .pptx

https://opendocumentformat.org/aboutodf/

[aumouvantsillage]

For reference, we had a conversation on this topic a few years ago, when the export feature was implemented as a separate command-line tool:
sozi-projects/Sozi-export#25