All galleries and singlepics

This series of commits makes the plugin
* move all pictures from all NGG galleries (not just the ones inserted into posts) to the media library. That also ensures that every image is moved only once, even if e.g. it's part of a gallery that's inserted into more than one post
* replace nggallery shortcodes in posts accordingly, omitting images that have NGG's 'exclude' property set.
* replace singlepic shortcodes in posts accordingly.