use a single QML engine

Summary:
use a shared qml engine for all ConfigModules in the same process
this avoids some crashes as creating and deleting stuff from
different engines is known to cause crashes in the v4 garbage collection
and is recomended to use a single QQmlEngine per process

Test Plan:
normal SystemSettings seems unaffected, plasma mobile systemsettings
which is qml-only now doesn't crash anymore after loading a module
for the second time

currently ported modules used on desktop are

• desktop theme
• look and feel
• sound
• splash screen
• boot splash

they all seem to work correctly
i also invite others to test them

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D8436