Add support for plone.app.imaging scales

this pull request is based on https://github.com/Raptus/raptus.article.gallery/pull/2 and requires https://github.com/Raptus/raptus.article.images/pull/1

the gallery image sizes can be defined as plone.app.imaging scales now

if plone.app.imagecropping is installed and the scale is croppable we show a cropping icon in the componentview