Including LayerShellQt in Plasma in time for 5.22

[Aleix Pol]

Hi,
In Plasma we have wanted a way to work with wlr_layer_shell for a
while, I'd tried taking this weird route, where we were adding it into
the project ad hoc, but it didn't really go anywhere.
https://invent.kde.org/plasma/kscreenlocker/-/merge_requests/20

I then figured it would make sense to find a make sense to just go
with what we need and implement it as a separate auto-contained
component that can be used by any client:
https://invent.kde.org/apol/layer-shell-qt
Here's what it would look like when it's used by KScreenLocker:
https://invent.kde.org/plasma/kscreenlocker/-/merge_requests/20

I'd like to port KSplash to use it and possibly some other components
it would make sense to get them there as well like KRunner or some
plasma shell bits.

The approach is a bit limited by QtWayland where it only supports
having one shell per process. I hope to work upstream to improve the
situation, this is a first step.

So, I'd like to get it into the next Plasma release, I trust we can
polish whatever might be necessary before 2021-04-29, the Repo freeze.

I've requested the move to kdereview, for those of you who can see:
https://phabricator.kde.org/T14327

Best regards,
Aleix

PS: Not an April fools', no :)