[Gzz] 2nd (Benja)

[Benja Fallenstein]

- Fixed the pagespan saving bug. (Actually, it was not a saving, but a 
loading bug :) )
- Abstracted out FallbackBinder API (now gzz.client.FallbackBinder is 
the API and gzz.view.DefaultBinder the default implementation).
- Implemented functionality for re-using a vobscene (so that a binding 
triggered on each mouse move doesn't need to incur the overhead of 
re-generating the whole scene, but can modify an existing one). Use 
Fallback.Win.getCachedVobScene() and 
Fallback.Win.setUseCachedVobScene(true) (see javadoc).
- Allow views to have their own binders, passed to Fallback in the 
constructor: there's a Map from views to FallbackBinders, and if a 
window's current view is a key in that map, the corresponding binder is 
used. Adding a new view and/or binder is done by modifying Gzz.run() in 
Gzz.py.

PROBLEM: the view-specific binders are selected by the view of the 
window that recieved the input focus, i.e., was front.

When writing view-specific binders, please take care that switching the 
view still works (maybe easiest: dispatch unhandled events to 
Fallback.binder (the default binder)).

- Benja