Notifier: proof of concept
Enabled a general mechanism for plugging in field observers.
Fixed broken filter tests introduced by observers.
Created explicit observer related modes for the dirty checker so that we
don't have to use the identity mode, which can break some scenarios.
Added tests for ObserverSelector interaction.
Added tests for Observer interaction.
Removed the change_detection source which contained the interfaces originally intended for multiple implementation but which are no longer needed.
First pass at merging DCCDGroup and WatchGroup
Beginning some cleanup and further simplification.
-Starting to remove unneeded getters.
-Removing old dccdgroup code.
-Removed unnecessary getters from dirty checking.
-Removing unneeded getters from AST.
-Improving code paragraphing.
-Removing unneeded code from watch record.
-Fixing linting errors.