[Task Manager] Fix Unity launcher count badge rendering when task is created

When moving a window that currently has a count badge between screens,
the newly created task on the other screen will have a glitched out badge overlay.

The anchors.fill didn't reliably work with the Loader, it seems it tries to deduce
a size from the item it loaded which is rubbish.
Forcing a width/height explicitly fixes this.

Differential Revision: https://phabricator.kde.org/D3318