caldav: add support for more props on MKCOL and PROPFIND

[DeepDiver1975]

trafficstars

Todo:

• [x] add support for these props on PROPFIND
• [x] build the same for CardDAV MKCOL

[DeepDiver1975]

@emersion please review

[DeepDiver1975]

@emersion how can we get this moving? THX

[emersion]

Some more general notes:

• Please try to keep 1 commit (or 1 pull request) per separate feature
• We don't use the "feat:" commit prefix, instead we prefix commit messages by the name of the Go package involved.

[DeepDiver1975]

@emersion @bitfehler please review again - did not yet change the color prop name ... as explained above this is the property I see in real life .... :shrug: