Asset Browser: Optionally restrict the search to a single folder

[arthurperton]

Fixes #6275.

Pretty straightforward. I have one thing I am not sure about though. In BrowserController@search, will it be okay to add the where clause $query->where('folder', $path) when a search index is used?

[jasonvarga]

will it be okay to add the where clause $query->where('folder', $path) when a search index is used?

Yup