Delay setting the model on the view until listing is finished.

Summary:
This massively cuts down on churn when instanciating a Folder View
configured to a large folder as the changing data set causes the
view to do a large amount of costly work.

A 7x6 Folder View pointed at /usr/bin would block Plasma during
startup for anywhere between 1-4 seconds depending on disk caches
(on a SSD). Now startup is hitch free, and even listing completes
much faster without the view slowing things down.

The widget is set to busy while listing.

Depends on D4343.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4344