Add previewPossible field to CamItemInfo that is used to decide whether the thumbnail is being queried from the backend.
This speeds up loading of thumbnails as it'll just use the mimetype icon when the backend says the thumbnails are not supported.
This is especially crucial for gphoto2 backend as it'll otherwise throw a lot of errors and slows down remarkably. Also probably speeds the UMS backend too as we don't try to parse the unsupported files.

Note, that CamItemInfo's operators were not changed except the debugging one.

CC