Fallback to in-process prompt if logout prompt fails

Summary:
KSmserver is currently very heavily tied to the shutdown prompt dialog.
On upgrades a user ends up in a situation where they have an old
ksmserver running but a newer shutdown dialog. This has caused issues
repeatedly in the past.

I want to make some changes to how the shutdown prompt is shown for
5.15. First step is making the old releases more robust.

Prompt looks ugly but a user will only see it at most once. Hopefully
never.

Test Plan:
rm'd the greeter
Got an ugly messagebox instead of nothing

Reviewers: Plasma, broulik, ngraham

Reviewed By: Plasma, broulik

Subscribers: ngraham, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15869