Solid-device-automounter/kcm: Use KConfigXT in ui

Summary:

• Make default button work
• Move connection from cpp to .ui
• Slight label change

Test Plan: Use default button

Reviewers: ervin, ngraham, Plasma, bport, crossi

Reviewed By: ervin

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27127