Makes the basics of ImageFolderModel works

Summary:
Refs T6421

• "By Folder" navigation works now
• Selection is also possible in "By Folder" view
• On clicking or selecting a folder( which is distinguished by the AlbumDelegate) a new column is pushed to the pageStack
• "By Folder" shows mixed folders and images that are navigable in hierarchy
• SelectedRole from the ImageFolderModel is removed as the ImageFolderModel is wrapped inside that of the SortModel which already implements SelectedRole

Reviewers: Koko, mart

Reviewed By: Koko, mart

Maniphest Tasks: T6421

Differential Revision: https://phabricator.kde.org/D6457