introduce inApps[] filter for search via ajax query

Make file results show up in files app only, allows limiting search results to specific apps. see https://github.com/owncloud/core/issues/821

Gracefully handles old apps that don't set the apps option for a search provider.

@PVince81 do we have a JS api to get the currently active app? I currently use `var classList = document.getElementById('content').className.split(/\s+/);` and grab the first class starting with 'app-' ... but that seems hackish to me.

PRs for contacts, calendar and other apps to follow, as well as a PR to move results to the content area instead of the current tiny dropdown overlay. 
`!empty($this->providers)` ?