[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:

Reviewers: cblack, VDG, Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

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