Widgetexplorer - wiring up

- add explorer subdirectory to shellpackge, add widgetexplorer filedefinition
- hook up containment added with a connection to showing the widget
explorer
- memory management for widget explorer: create on demand, delete after
close: Maybe we want to cache it? For now, go for reduced memory
consumption rather than quick opening, and see how slow this might
react.
- add QQuickView for widget explorer in desktopcorona

With this patch, I can open a widget explorer window from the QML
containment. Of course, the widget explorer is just a pink window right
now.