loader: allow to set the parent of the subsystem's view

This commit adds a 'parent_view()' function to the loader session, which
allows to set the parent view of the subsystem's Nitpicker view.

If the function is to be used, this must get done before calling
'start()'.

Fixes #1172.
Qt5: set the parent view of a QPluginWidget's view

For the correct integration of a QPluginWidget in a parent QWidget, with
this commit the parent QWidget's Nitpicker view is made the parent view of
the plugin's Nitpicker view.

Fixes #1173.