Mark non-persistent notifications as transient.

Summary:
On Gnome and other desktops using their extension of the notifications
spec, notifications are persistent unless they set the transient hint.

This tries to make notifications more consistent across desktops.

Test Plan: Manual testing on Gnome.

Reviewers: Frameworks, Plasma, apol

Reviewed By: apol

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D4213