file: Generate thumbnails when the preview icon is available

Currently, thumbnails are not generated for MTP/GPhoto2 locations. The backends return G_FILESYSTEM_PREVIEW_TYPE_NEVER in order to prevent thumbnailers to make them unresponsive. However, the backends provide preview icons for some file types and thumbnailers should prefer those icons if available. Let's generate thumbnails for files when the preview icon is available. This should not affect backend responsiveness much.

Fixes: #1921 (closed)