This diff sets the initial focus of the internal previewer to the Part widget and sets the autodefault property of the Close button to false. The latter prevents the Enter key from closing the dialog which allows the Part widget to handle the key press event instead.

Fixes two bugs:
Bug 369401
Bug 369390

Meant for 16.08 branch.

1. Open any archive and preview a file with internal previewer.
2. Check that the Part widget has focus.
3. Check that the Enter key doesn't close the dialog.