[refactor] [test] renamed base namespace from Events to Event
[refactor] [test] move event managers to Manager\Observable and Manager\Shared namespaces; move Handle to own namespace Handle; move HandleLsit to HandleCollection under own namespace HandleCollection
[refactor] [test] refactored exceptions to own namespace and using a library marker interface
[test] previously not covered UndeclaredEventException in ObservableManager
Merge branch 'develop'