[mediacontroller applet] Reorder elements

Summary:
Improve overall look according to bug report and mockup.

BUG: 390280

{F5741858}

Reviewers: #plasma, andreask, broulik

Reviewed By: #plasma, andreask, broulik

Subscribers: ngraham, abetts, nicolasfella, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11063