preload the expander (the dialog) too

Summary:
when preloading, preload the expander as well, and reparent the
full representation to it, so we have a scene schanged less.
also do a create() of that window (may be worth to do create()
automatically in every Dialog?)

Test Plan: the first open is visibly a lot faster now

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D10681