Add "Make primary" button to pictures in asset view

[sczther]

It would be great to be able to easily change which image is primary while browsing assets. In the view of the asset, you can see files on the right of the window, with the primary picture on top and bigger. Each picture has a edit pencil icon in the corner. I would like a star icon in the corner of each image, with it being colored in for the primary image. Clicking the star would make the image a primary one.