Continued and refined work on bindings.

The BG binding approach was canceled and, instead, BG bindings where
split into two bindings:
- Background getters, wich observe background changes and pass on the
new values
- Background setters, which observer incoming values and passes these on
as background set requests
Background getters/setters can be generalized further into property
getters/setters.

Further, model may be dropped onto other models, which they observe.

These changes make it possible to "program" a one-way-mirroring on
background changes among two models.

Signed-off-by: Jakob Brandsborg Ehmsen <jakobehmsen.github@gmail.com>