[mobile/wifi] Use ScrollViewKCM

Summary:
This gives the KCM a proper frame/scrollbar

Needs D27562 to not always show the error message bubble

Test Plan:
Before:
{F8122104}
After:
{F8122107}

Reviewers: #plasma, jgrulich!, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27563