Support for Search API based views in the Media Browser

Hi,

We have a semi-large (~11.500) media-library with meta-data fields on (photographer, description, etc). So we use a Search API index of media-items to let our users search through these.

We would love to use a view based on the index in the Media Browser, but because the display- and style plugins only register file_managed as their base, this is impossible. After tweaking the base, I found a few other places where the SQL- and Search API based Query plugins differ, so two other places needed some checks too..

But attached is a patch that enables the Media Browser to use Search API based views as well.

Regards
Morten