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.
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082