[imageplugin] strip layout elements on save

[dbu]

in #36 we merged the WIP for the image plugin to hallo. this is one of the tasks that remain to do:

on save, the drag+drop elements should be stripped out of the content that is stored through the backbone. currently the ui-draggable attributes and with bad timing even the blue blocks get stored.

[bergie]

The right way to do this is to provide a cleanupContentClone method in the plugin. See for instance how annotate plugin does it.