#67 created a panel to show the preview images

Created RepresentationPreview, this panel is able to show the image
behind a content location of a representation.
 #67 added the 'Data Preview' panel to the resource renderer

In the 'Data Preview' panel, TaskPanes for those representations are
added whose content type is an image. The content type is checked with a
HEAD request.
 #67 create preview panel only for preview data

Create preview panel only for representations of the type 'preview data'
 #67 set the title of the taskpane
 #67 resize the image in RepresentationPreview
 #67 added a size reference to RepresentationPreview

RepresentationPreview needs a size reference, because the component does
not know how tall it can become when it scales the image.
 #67 scale the image immediately after it was fetched
Merge branch 'feature/67-add-panel-data-preview' into dev