Image browser support for ATReferenceBrowserWidget

Hi,

I've made a small extension to ATRBW that allows you to preview
images to be referenced within the widget:

 - you can specify two new properties 'image_portal_types' which is
   is a list of image portal types for which instances will be shown
   within the widgert and 'image_method' allows you to specify a method
   name that is used to preview the image (e.g. 'mini' for thumbnails).

Any objections merging this extension to the trunk?

Andreas

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