mainwindow: Make the version display more robust in the about window

Also add a unittest for the latest version logic and
moved the Pitivi tests to test_application.py because
that's the module where the Pitivi class resides.
tests: Move the system module tests to test_system

Also some cleanup in the file headers.
tests: Rename test_base.py because it's not a test
render: Allow the render progress dialog to close the desktop notification

This got broken in 38861f742af4ce115c9265f3e7c85d8ea0f17111