Update model watcher dsl to support sealed classes

[ShikaSD]

Achieved my delegating subclass diff implementation to other instances of model watcher.

• Added clear method as a part of public API of ModelWatcher<T>.
• Added type and objectType DSL methods for ModelWatcher.Builder<T>

Example:

modelWatcher<Model> {
    type<Model.A> {
        Model.A::field { 
            // Do stuff
        }
    }
    objectType<Model.B> { modelB ->
        // Do stuff
    }
}

Fixes #111

[SavinMike]

Maybe better to change it to clearChildren method?