Add `ol.ObjectEvent` for changes to `ol.Object` properties. Previously, `ol.Object` dispatched instances of `goog.events.Event` with type `change`. Now `ol.ObjectEvent` instances will be dispatched on property changes. The events include a `getKey` method to get the name of the property being changed. The `beforepropertychange` type event is fired before a property value changes, and the `propertychange` type event fires after the property value changes.
Where an enum value is used as an event type, it should be alllowercase (to follow DOM events). Property names should be ALLUPPERCASE in this case (just as camelCase and PascalCase are converted to CONSTANT_CASE).
Any event target can be used to dispatch generic goog.events.Event instances with an arbitrary type. In cases where we dispatch custom events, we should not use type values that collide with those used for generic events (at least internally). This allows listeners a better chance of knowing what kind of argument they will receive.
As subsequent change will clean up the enumeration and add a bit more consistency.
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.
Layer renderers should not be responsible for listening to layer
properties change and triggering a render.
Layer change events are now forwarded to the map which will trigger a render.
