Improve share access process and share examples

[glpatcern]

This PR is an attempt to fix #157, but in doing so it partly breaks the v1.1 protocol to access a share.

In particular, for the webdav and datatx protocols compatibility is ensured by keeping the same property name uri (resp. srcUri), but the recommended value is now a simple identifier or "key", not a full URI.

For the webapp protocol I propose to break compatibility, assuming that the only implementation that requires to be adapted is Reva.

This is still work in progress as I'm myself not fully happy, the main outstanding question being shall we actually drop uri and go for sharedKey?

[mickenordin]

I think breaking backwards compatibility for webapps should be ok, the one I was thinking about except reva was Nextcloud talk, but they don't use webbapp, but rather talk-v1:

    {
      "name": "talk-room",
      "shareTypes": [
        "user"
      ],
      "protocols": {
        "talk-v1": "/ocs/v2.php/apps/spreed/api/"
      }
    }

[glpatcern]

That's right, talk is a "protocol" on its own, and I can obviously take responsibility to align Reva's webapp support to OCM 1.2 if we go ahead with this PR.

[michielbdejong]

LGTM

[michielbdejong]

I think it's OK to leave it as "uri", with the note that it SHOULD be a relative one.