port the ksplash kcm to the new design

Summary: redically simplifying the code, port the ksplash kcm to the new design

Test Plan: selecting and preview work correctly, resize behavior is correct

Reviewers: #plasma, #vdg, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: ngraham, abetts, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12088