more control on what shell is loaded

Summary:
Since shellManager was removed, we have a quite big regressions
of any other shell that isn't the desktop one not loading in any
way from SDDM, causing a big issue for plasma mobile or any shell
based on plasma mini shell, which we have several now not working
images based on.

this allows for a shell being configured in plasmarc

Test Plan: shell specified in plasmarc loaded, loads the desktop if not specified

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23031