Add method to ModelContext for application driven observation #229

Currently the spec only mentions

The times at which a browser agent performs an observation are implementation-defined

But to me it seems that in practice it would be the app itself that is best positioned to perform an observation, since only the app dev knows what part of the app state would most relevant to a browser agent, and therefore it can send observations when that state changes and avoid filling up the context of an agent with irrelevant stuff.