queue load() after all deleteLater() in unload() are done

Summary: Put load in queue of the event loop to wait for the whole set of containments to have been deleteLater(), as some like FolderView operate on singletons which can cause inconsistent states

Test Plan:
now switching look and feel between 2 themes that use both FolderView
as desktop containment, the icons are still there

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21513