Small refactoring for scrolling, includes renamings.


git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@966 ef8490bc-f317-4725-81ed-e95a9aca1bcf
A first small step to overcome the double page limitation. (Refactoring)

Changed method signatures:
- ImageHandler.get_pixbufs
- ImageHandler.get_pixbuf_auto_background



git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@967 ef8490bc-f317-4725-81ed-e95a9aca1bcf