Fix splash screen scaling when using QT_SCALING for selective hardware and resolutions
- A special usecase for devices where we don't / can't support 4K resolution like the Raspberry Pi 4
- Using PLASMA_USE_QT_SCALING with QT_SCREEN_SCALE_FACTORS causes a bug where Parent ScreenGeometry does not change when applying resolution changes on boot from 4K to 1980x1800 resolution via Kscreen-Doctor
- As Ksplash does not see the ScreenGeometry change we need to manually listen for config application changes and Kscreen DBUS signals are unreliable at boot hence need to implement our own listening channel that will always be available
- This does not affect / change behaviour on any other device except for special use cases like hardware mentioned above in combination with 4K resolution TV's