Modernize and redesign the Night Color config window

Summary: Port to Kirigami FormLayout and QQC2.

Test Plan:
Before:
{F6878283}

After:
{F6886140}

{F6886139}

{F6886138}

Reviewers: #vdg, ngraham, romangg, mart, #plasma, davidedmundson

Reviewed By: #vdg, ngraham, #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Maniphest Tasks: T10273

Differential Revision: https://phabricator.kde.org/D21668