Cherry-pick to fix layout mutability regression

[Nate Graham]

Hello faithful distro packagers!

A major regression was found in 5.18.0 for users who had previously 
locked their widgets: https://bugs.kde.org/show_bug.cgi?id=417424.

The bug has already been fixed, and I would like to encourage everyone 
shipping a rolling release distro to cherry-pick 
https://cgit.kde.org/plasma-desktop.git/commit/?id=2bc3c5e92d4789146548e8de4d520cd191994e1c 
so users don't have to wait for Plasma 5.18.1 to get it fixed.

Thanks everyone!

Nate