Initial implementation of diagram-as-element (embedding diagrams)

TODO:
 - extend the renderer interface to get rid of the extra DiaImage,
   at least for some renderers (DiaRenderer::set_matrix?)
 - the image gets updated too often, something wrong with mtime check?
 - make diagrams directly selecteable in the properties dialog file entry
 - allow to open the diagram stand-alone by context menu
 - make new object available in the Misc sheet