[effects] Show a notification when a screenshot got saved to file

Summary:
Inform the user about the path to where a screenshot got saved through a
notification. This is helpful on the one hand that the user sees that
the screenshot got saved and from a security perspective that the user
is informed when a screenshot is taken through the dbus interface. It
doesn't prevent non-authorized screenshot taking, but at least the user
is informed about it.

Reviewers: #kwin, #plasma, #vdg

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3376