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/dom/input.js

@@ -30,6 +30,10 @@ ol.dom.InputProperty = {
  * @extends {ol.Object}
  * @param {Element} target Target element.
  * @todo stability experimental
+ * @todo observable value {string} the value of the Input
+ * @todo observable valueAsNumber {number} the value of the Input, converted to
+ *       a number if possible
+ * @todo observable checked {boolean} the checked state of the Input
  */
 ol.dom.Input = function(target) {
   goog.base(this);