Port mobile_wifi kcm to newer APIs

Summary:

• Controls.Button can display icons on its own now
• Use kcm.push() and kcm.pop()
• Use Kirigami.ListSectionHeader instead of custom code

Reviewers: jgrulich!, nicolasfella, Plasma:_mobile

Reviewed By: nicolasfella, Plasma:_mobile

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23897