Restore asynchronous QML object incubation in krunner

Summary:
Using KDeclrative::QmlObject::rootObject before it's created blocks the
thread until it finishes so that code always "works" - but displays a
warning and is slower.

We now no longer get a warning on startup.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2086