plugins/sessionfade: Improve visual flow into the session

• Adds an animation at startup that fades in as soon as the first window is shown. This way there's no sudden appearance of the splash screen or the desktop.

Now when locking the screen we'd shun the desktop going to back for a few ms then lockscreen. Instead:

• Adds a fade to black and waits for the lockscreen to appear then fades into the lockscreen.
• After unlocking, it blends into the desktop again.

before: master

after: applied

The screentransform shader for the lockscreen blending (i.e. startFadeIn(true)), also there's opportunity to share some code if we care.

Note our common strategy for fading out doesn't apply here because since we hide the windows we lose the window anyway.

Depends on kscreenlocker!214