[WIP] 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 ... but there needs to be an extendable way how to do that.