Save an object per panel applet

Summary:
Instead of Item { Loader {} } just make the applet
container the Loader. Also saves some prop bindings
for sizing and centering the BusyIndicator, which
still appears correct.

Some props marked read-only as a bonus.

Reviewers: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9152