Corrects logic to distinguish among foreign path and ~/Pictures path at the startup of the application

Summary:
Refs T6498

• the earlier logic to distinguish between foreign path and ~/Pictures was in setUrl() of ImageFolderModel, which was called again again every time any folder is clicked and thus setting the url again to ~/Pictures

Reviewers: Koko, mart

Reviewed By: Koko, mart

Maniphest Tasks: T6498

Differential Revision: https://phabricator.kde.org/D6753