Revert "MainWindow: Delete navigationwidget again"

This causes trouble with the unit tests on windows.

This reverts commit 0fe28d652f1219978d23030cbb322d09ada5a68f

Change-Id: I08d4991d9b86ee8b80d093899f2e33e2be1c1565
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>