This avoids having to completely load the quicksettings, status bar,
media widget, and notifications list from scratch when the view changes.
We now have a single instance of each component which is reparented to
the new view (portrait/landscape).
Fixes https://invent.kde.org/plasma/plasma-mobile/-/issues/406
Move from a C++ library + QML plugin to a QML plugin only for simplicity, since the homescreen switching architecture will be done from Plasma, and so use of the shell library only needs to be from QML.
