Support RFC 3253 (Versioning Extensions)

[jwodder]

RFC 3253 seems to be largely concerned with resource version history, but it looks like there may be some things in there that are more generally applicable.

[yarikoptic]

For the read-only operation, as what we have here ATM, what versioning aspects besides providing ETAGs (for files only or also for collections/folders where available) are "in play"?

[jwodder]

@yarikoptic

• For what's currently implemented, the only things that could be viewed as versioning-related are etags and "modified" timestamps.

• As regards RFC 3253, I haven't looked at all of it yet (that's part of this issue), but it seems to define some WebDAV features that dandidav could support even without actual versioning, like a <DAV:supported-live-property-set> element (though, looking further into this at the moment, it seems that particular example is only allowed if versioning is fully supported).