mainwindow: Reuse in_devel from configure

Also fix its usage.
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