Slideshow opened from files view now loads images from gallery

When opening the slideshow from the files view, the image URL is
converted into a gallery URL. This makes sure it uses the cache and
correctly rotated imaged.

Previously it was using the download URL.

Fixes #1462
Merge pull request #1469 from owncloud/slideshow-loadfromgallery

Slideshow opened from files view now loads images from gallery (fixes orientation)