Fix loading the lockscreen fallback UI
kscreenlocker has a fallback UI loaded if the main QML from the LNF fails.
As the main code is in a loader the lockscreen /always/ loads even if the main contents can't be loaded. It also contributes to a black flicker when loading the lockscreen.
The rationale for the loader was to make the window appear faster. This hopefully isn't needed now we have the logind integration delaying the sleep.
If it turns out this is feature desired it would be far more productive to do this in the greeterapp c++ code. Show an empty QtQuick window and then set the source on it later. It'd be far faster which would help towards the original goal and allow us to check the loading.
This patch also removes the opacity fade as there's already an opacity fade on contents when one moves the mouse when the "screensaver" like code looks smoother.