Dav response parser & possible usage of existing dav libraries

[lookacat]

With the work on https://jira.owncloud.com/browse/WEB-1095 we have found some issues regarding our dav.js.

1. Parsing of e.g. Propfind response does manipulate the keys und values of a prop significantly. This lead to information being thrown away. Only limited value types are supported and more complex values are either parsed wrong or are just cut off. It's also really time consuming to extend the parsing logic, and it's hard to understand existing code. E.g _parsePropNode in dav.js is not providing much value and is really hard to extend.

2. We've discussed why we did write our own dav client if there are some promising libraries out there that are well maintained and would save us time and effort. Libraries we came across are:

• https://github.com/perry-mitchell/webdav-client
• https://tsdav.vercel.app/

3. We should consider switching to an external library but we need to make sure they suit our needs and fit in with our code.

Issues that will be resolved by this:
#1032