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.

BUG: 370676
FIXED-IN: 5.25