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/map.js

@@ -143,6 +143,12 @@ ol.MapProperty = {
  * @extends {ol.Object}
  * @param {ol.MapOptions} options Map options.
  * @todo stability experimental
+ * @todo observable layergroup {ol.layer.LayerGroup} a layer group containing
+ *       the layers in this map.
+ * @todo observable size {ol.Size} the size in pixels of the map in the DOM
+ * @todo observable target {string|Element} the Element or id of the Element
+ *       that the map is rendered in.
+ * @todo observable view {ol.IView} the view that controls this map
  */
 ol.Map = function(options) {