We no longer add observable annotations to the constructor.
Instead, we just mark getters (and for read/write properties
also setters) with an observable annotation.
This change adds a stability value to the api annotation, with
'experimental' as default value.
enum, typedef and event annotations are never exportable, but
api annotations are needed there to make them appear in the
docs.
Nested typedefs are no longer inlined recursively, because the
resulting tables get too wide with the current template.
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.
