Fix DBus service activation
Previously this was supposed to be started in ElisaApplication::create, except it wasn't started because this function was never called.
https://doc.qt.io/qt-6/qqmlengine.html#QML_SINGLETON
By default, each QQmlEngine will try to create a singleton instance using either the type's default constructor or a static factory function of the signature T *create(QQmlEngine *, QJSEngine *) when the type is first accessed. If both do exist and are accessible, the default constructor is preferred.
Test-plan:
I'm not able to reproduce the attached bug report, but I was able to fix the following issue:
Run kdesrc-build --run elisa. After the window is launched, run the same command in a separate terminal session.
Before: two instances of Elisa opened
Now: only one instance of Elisa opens.
Amends fc374017 BUG: 487905