Support templates with non-empty content

[jaakkor2]

I get error

ERROR: LoadError: input template pptx already contains slides, please use an empty template

We could replace line https://github.com/ASML-Labs/PPTX.jl/blob/v0.6.0/src/write.jl#L19

        error("input template pptx already contains slides, please use an empty template")

with

       rm("slides", recursive=true)`

[matthijscox-asml]

We could, but then we are silently removing slides from the user. I'd rather have them delete the slides manually, while we in the future make it possible to keep those original slides and add new slides to the slidedeck.

Ideally, we make it possible to edit the original slides, but that requires me to build a complete PPTX data model, which would be a lot of work.