Add @todo observable documentation for observable properties

This PR adds documentation for observable properties, which will then be pulled into the docs correctly once #1180 is merged.  This is a first pass based on searching for definition of observable properties being defined as enums after lines ending with `Property = {`.  If there are observable properties implemented that don't follow this pattern then they are not included.

I've added simple descriptions based on what I know or could easily figure out, there may be some properties (like preload) that are not correctly described.

I've also added `readonly` annotations where I knew that a property was readonly.  I may have missed some readonly properties.

ol.layer.Base has a bunch of properties but I don't think it is exported so the documentation of these properties will not show up, so I added the documentation to ol.layer.Layer instead even though this isn't really where it should be documented.

src/ol/control/mousepositioncontrol.js

@@ -38,6 +38,10 @@ ol.control.MousePositionProperty = {
  * @extends {ol.control.Control}
  * @param {ol.control.MousePositionOptions=} opt_options Mouse position options.
  * @todo stability experimental
+ * @todo observable projection {ol.proj.Projection} the projection to report
+ *       mouse position in
+ * @todo observable coordinateFormat {string} the format to render the current
+ *       position in
  */
 ol.control.MousePosition = function(opt_options) {