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