Reduce spurious signal emissions

Summary:
We were reporting that the margins were changing whenever anything changed
in a FrameSvg, this patch makes sure we are only reporting the changes
when it actually changes.
This also fixes a binding loop in Desktop.qml from plasma-desktop.

BUG: 382233

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: broulik, davidedmundson, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D8809