Update this repository with what is in the Tracim repository

[raphj]

After my modifications in Caldavzap at tracim/tracim#5359, I've looked into updating this repository.

But both code have already diverged so this is not a straightforward operation.

How this repository is handled should be clarified in some documentation somewhere, with our expectations, and then the repo should be updated accordingly.

@lebouquetin @Skylsmoi @buxx

[buxx]

I think the Tracim caldavzap source code can replace this repository code. But as a tracim part code because caldavzap code has been tweaked for Tracim backend.

Or, we can consider this repo is obsolete and keep caldavzap code in Tracim repository.