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/layer/layer.js

@@ -14,6 +14,14 @@ goog.require('ol.source.Source');
  * @extends {ol.layer.Base}
  * @param {ol.layer.LayerOptions} options Layer options.
  * @todo stability experimental
+ * @todo observable brightness {number} the brightness of the layer
+ * @todo observable contrast {number} the contrast of the layer
+ * @todo observable hue {number} the hue of the layer
+ * @todo observable opacity {number} the opacity of the layer
+ * @todo observable saturation {number} the saturation of the layer
+ * @todo observable visible {boolean} the visiblity of the layer
+ * @todo observable maxResolution {number} the maximum resolution of the layer
+ * @todo observable minResolution {number} the minimum resolution of the layer
  */
 ol.layer.Layer = function(options) {
 

src/ol/layer/layergroup.js

@@ -28,6 +28,8 @@ ol.layer.GroupProperty = {
  * @extends {ol.layer.Base}
  * @param {ol.layer.GroupOptions=} opt_options Layer options.
  * @todo stability experimental
+ * @todo observable layers {ol.Collection} collection of layers that are part
+ *       of this group
  */
 ol.layer.Group = function(opt_options) {
 

src/ol/layer/tilelayer.js

@@ -18,6 +18,7 @@ ol.layer.TileProperty = {
  * @extends {ol.layer.Layer}
  * @param {ol.layer.TileOptions} options Tile layer options.
  * @todo stability experimental
+ * @todo observable preload {number} the level to preload tiles up to
  */
 ol.layer.Tile = function(options) {