Makes the Thumbnail use the thumbnailer, common to all the models

Summary:
Refs T6422

• Creates a RoleNames enum in Roles class which is kept common for every model
• Thumbnail generation code is in SortModel now
• ThubnailRole uses the ImageUrlRole of every model to generate thumbnails for the image.
• The CoverRole of the ImageLocationModel and ImageTimeModel has been changed to ImageUrlRole
• ImageUrlRole for ImageTimeModel and ImageLocationModel returns the url of the first image in the collection ie the cover image
• A little modification has been done in ImageStorage's functions to add "file://" prefix to the urls it returns for the ImageTimeModel and ImageLocationModel

Test Plan: testing has been done for all the models

Reviewers: Koko, mart

Reviewed By: Koko, mart

Maniphest Tasks: T6422

Differential Revision: https://phabricator.kde.org/D6478