[KPropertiesDialog] provide a way of showing the target of a symlink

Summary:
BUG: 413002
FIXED-IN: 5.65

Undecided on the button. Should it be text-only displaying "Show" instead of an icon?
Or text-and-icon? Is there a better icon we could use or is this one good enough?

Test Plan:

Reviewers: VDG, Frameworks

Subscribers: apol, ndavis, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25226