[research] search API, webdav rfc5323 approach

Following the discussion in https://github.com/owncloud/core/issues/12504 and https://github.com/owncloud/core/issues/12543 I stated to work on a search API that is *loosely based* on [RFC5323](http://tools.ietf.org/html/rfc5323). I got as far as being able to use our search providers and even limiting results to a specific app. Unfortunately, we need to adapt the current search result object properties to webdav properties. For files that can be done with `$this->server->getPropertiesForPath` [see diff](https://github.com/owncloud/core/pull/12884/files#diff-6ec5b90324d958c357b630b3b6744374R162) but there needs to be an extendable way how to do that.
phpdocs please