[applets/mediacontroller] Make the right margin the same as the left margin

Summary: Now the song text can't almost touch the right edge.

Test Plan: {F8256425}

Reviewers: cblack, #vdg, #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29143