Get rid of MaxWidthImageButton

Summary: There is no need to implement a custom view for that. It is only used in MPRIS Activity, which looks almost the same now, except that the play button aligns with the buttons beneath, which is better IMHO.

Reviewers: KDE Connect, mtijink

Reviewed By: KDE Connect, mtijink

Subscribers: KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D9520