Switch to KNSQuick for getting new wallpapers

Summary:
This switches the image plugin to using KNewStuffQuick instead of
launching the widget dialog through that ol' widget dialog hack.
It further uses the big preview mode by default, which just seems
to make a lot of sense for wallpapers.

Test Plan:
Without: Uses the old dialog
With: Uses the new dialog, as seen below
{F7882885}

Reviewers: mart, #plasma, ngraham

Reviewed By: mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26665