WMSGetFeatureInfo API enhancements

This pull request suggests to promote several properties of the `WMSGetFeatureInfo` control to the public API. 

The properties

  * layers
  * queryVisible
  * url
  * layerUrls
  * infoFormat
  * vendorParams
  * format
  * formatOptions

should all be visible in the public API since they are essentially the only way to customize instances of this control in a sane way.

Thanks in advance for any review.

(Related: pull request #871 already added some missing APIproperties to the `Measure` control.)