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/deviceorientation.js

@@ -69,6 +69,16 @@ ol.DeviceOrientationProperty = {
  * @extends {ol.Object}
  * @param {ol.DeviceOrientationOptions=} opt_options Options.
  * @todo stability experimental
+ * @todo observable alpha {number} readonly the euler angle in radians of the
+ *       device from the standard X axis
+ * @todo observable beta {number} readonly the euler angle in radians of the
+ *       device from the planar Z axis
+ * @todo observable gamma {number} readonly the euler angle in radians of the
+ *       device from the planar X axis
+ * @todo observable heading {number} readonly the euler angle in radians of the
+ *       device from the planar Y axis
+ * @todo observable tracking {boolean} the status of tracking changes to alpha,
+ *       beta and gamma.  If true, changes are tracked and reported immediately.
  */
 ol.DeviceOrientation = function(opt_options) {