[wallpapers/image] Disambiguate identical folder names in paths list

Summary:
When multiple paths that end in the same folder name are added (for example with D26483),
currently the list shows multiple identical entries distinguished only by their different
tooltips. This patch improves the situation by putting the path to the folder in a
subtitle.

Test Plan:
Before:

After:

Reviewers: VDG, Plasma, mart

Reviewed By: VDG, Plasma, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26510