DiffEditor: Cosmetics

Add a bit of space around "Context Lines" label.
Remove extra spinbox frame.

Change-Id: I69533c2fa3e93473f41194bdc36ce07042df9855
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>
Make option for diff viewer in git options clearer

Change-Id: Iae0f427ed447430c82852c967022bc30ee4e88d1
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>