[applet] Fix button bar right margin

Summary:
Button row was incorrectly adding unnecessary extra right padding; see D28089 for details.

Depends on D28089

Test Plan:
Before: {F8180305}
After: {F8180304}

Reviewers: #vdg, #plasma, drosca, ndavis

Reviewed By: #vdg, ndavis

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28090