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