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Support plain JS and ko-es5 models (#55)

* POJO-Support: Compel knockout to generate _ko_property_writers for a few more of its bindings, by marking them as two-way.

Some of knockout's built-in bindings (checked, hasFocus, selectedOptions, textInput, and value) are already listed as "two-way bindings" by default. For these, knockout generates an additional binding named "_ko_property_writers" which exposes functions for writing these values to non-observable model properties.

We need to tell knockout to generate property writers for more of its built-in bindings, so that `init` can use these to write to the model.

* POJO-Support:  Added utility method for creating a binding preprocessor that generates custom property writers.

Knockout's _twoWayBindings collection only covers basic `name: value` bindings.
For more complex bindings that receive an object literal (including KO's own "attr"), we need to generate property writers ourselves.

* POJO-Support:  Modified `init` binding, to use property writers for non-observable model properties.

This commit expands on the existing logic of finding and invoking fieldAccessors. But if no fieldAccessor is found, or it is not observable, then look for a suitable property writer instead.

* POJO-Support:  Modified `foreachInit` binding, so it can work with plain arrays, and also subscribe to changes in a knockout-es5 tracked array property.

The trickiest part of this commit is setting up the subscription with change tracking. When the array comes from a knockout-es5 "tracked" model, it is actually being read from an ES5 property getter which reads from an internal observableArray. But we don't have access to that underlying observableArray here, so we can't call `array.subscribe(..., "arrayChange")`.

Instead, we take advantage of Knockout's dependency tracking and set up a `ko.computed` which reads the array from `valueAccessor()`. This sets up a dependency such that the body of our `com... (continued)

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