Unified boot process

[plfiorini]

Currently we have a different login screen and lock screen but they must be identical in order to look coherent.
The whole boot process in Hawaii had the idea of transitions, here's an early prototype: https://www.youtube.com/watch?v=A-NbOK4ZDl8
It got refined but as far as the transition goes it's still the same.

The boot process goes like this:

1. Plymouth theme with greyish background and macOS like spinner
2. SDDM fades in with the same background
3. A QML transition kicks in and the greyish background is pulled up revealing the login prompt like a slideshow
4. When the user authenticates a still of the desktop or the user wallpaper is shown with the same slideshow effect
5. SDDM fades to the splash screen

The lock screen should look exactly like SDDM