Scrolling bug fix and refactoring

- When loading a new page in double page mode, the viewport now automatically
moves to the actual page rather than just the corner next to it.
- MainWindow.scroll_to_fixed replaced with scroll_to_predefined.



git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@982 ef8490bc-f317-4725-81ed-e95a9aca1bcf
More scrolling bug fixes and improvements

- various bug fixes concerning smart scrolling
- considerably improved smart scrolling



git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@983 ef8490bc-f317-4725-81ed-e95a9aca1bcf
Split scrolling.py (Refactoring)

- Split scrolling.py into scrolling.py, layout.py and box.py.
- Move constants in scrolling.py to constants.py



git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@984 ef8490bc-f317-4725-81ed-e95a9aca1bcf