Add ability to expose View controls

[philippjfr]

Implements #124

The implementation I settled on is very explicit, each View now has a controls parameter which declares which parameters on the view will be exposed as widgets. This can either be set via the yaml specification or in the case of custom views a default value can be set which will expose those parameters. I'm much happier making this explicit rather than relying on any precedence or constant declaration since the author of a View cannot really know ahead of time which parameters it makes sense to expose.

To make this work well for parameters which reference fields in a table each View must declare _field_params. This allows the baseclass to know that it should populate the available options on this parameter with the available fields in the table so that the generated widgets offer a dropdown letting the user select between the available fields.

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