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.
Now that it is possible to resize the stack of the Genode main thread, it
is not necessary anymore to create a new Genode thread as Qt main thread.
Fixes#1134.
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
