Changed the way how the main window is used.

* The main window widget is no more accessable from other modules
* Modules should use addWidget() and addLayout() methods to add widgets
  and layouts to the main window.
* Any widgets and layouts added to the main window are deleted in the done()
  method.

Fixes the issue where the main window widget tried to delete its children and failed, because
modules that created them were already unloaded.