introduce ApplicationItem

Summary:
This item, can emulate ApplicationWindow and
is supposed to be used in applications that for some reason
need the UI to be loaded in QQuickView

Test Plan:
tried the gallery to use an ApplicationItem instead of an
ApplicationWindow, seems to work in the same way

Reviewers: Kirigami, leinir, apol

Reviewed By: leinir

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D5166