correcting problem with accessing LayoutConstraints enum member values
- refactoring editor update to encapsulate all update-related logic within UpdaterImpl and let editor only manage setRootCell() process
- editor commands created in order to simplify editor selection update process
introcuding AbstractComputeRunnable class in order to be able to overwrite compute() method in addition to parametrizing ComputeRunnable with new "Computable" instance.
using (subclassing) EditorCommand instead of corresponding deprecated methods from EditorContext
Merge remote-tracking branch 'origin/master' into 3.2/editorUpdater