Add 'ignore' button

As mentioned in a previous bug report (432140), the current workflow of "ignoring" face tags is confusing. By now, you need to set a face as "unknown" first and afterwards it is possible to "ignore" it. Therefore, this patch implements a third button to "ignore" faces directly (as proposed by Jens:

'But in this case it should also be available as a third button while viewing the image with its face rectangles to make the user experience consistent. So we have buttons for "add", "ignore" and "delete" a face rectangle.').