fade to blur when the login box appears

Summary:
for both lockscreen and login screen:

• display the wallpaper and the clock with a shadow by default
• at the first mouse or keyboard input, make the input fields appear
• the actual controls appear pretty fast
• the wallpaper starts to blur, desaturate and fade to darker
• after 10 seconds make the controls disappear again

• as soon as anything is written in the password field never fade out the controls
• if the virtual keyboard is open, never fade out the controls
• if anything is pushed on the stack, like "switch user", never fade the controls
• Esc clears the field and makes controls disappear (closing keyboard if needed)

the fade won't happen if the background is a simple color

BUG: 369676
BUG: 388622
FIXED-IN: 5.13

Depends on D12314

Recommended D11309 and D11308 to change the default wallpaper to plasma wallpaper

Test Plan: tested the behavior of all the above points

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: zzag, abetts, davidedmundson, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11928