Motivation for this change

KDE software in NixOS is not at the latest version. This patch release-19.03 updates to

• KDE frameworks 5.57
• Plasma 5.15
• KDE applications 19.04

I've been running my desktop on a nix channel with this patch for a few weeks. So far I've observed only one bug (https://bugs.kde.org/show_bug.cgi?id=406829). I have not been able to look into the details of the bug because:

• I cannot get Qt 5.12.3 to compile to see if that fixes it (qt 5.12.0 -> 5.12.3 WIP #60119)
• I do not know how to install Qt package and up with debug symbols enabled.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• [~] Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.