Don't cause unnecessary resizes on containment change

When an applet is moved from a contianment to another, for a moment it was parented to the whole AppletsLayout.

since it had anchors.fill:parent set, it was resized to the whole appletslayout geometry or its max size if had any. for instance the kickoff applet would be resized to the whole screen

(cherry picked from commit b3a5eff5)

511616b5 Don't cause unnecessary resizes on containment change

Co-authored-by: Marco Martin notmart@gmail.com