Some modifications to make UnitySerializedDictionary more easy to use

[znyb]

1.add namespace
2.move UDictionaryExtensions to Editor
3. if key type or value type do not support serialization ,show error message in inspector and show empty list
4. remove UDictionaryReadOnlyDrawer and mark [ReadOnly] on the field instead
5. UDictionaryDrawer is now valid for all serializable subclasses inherited from UDictionary ，no need to assign attribute to [UDictionaryDrawer] or add wrapper Drawer
6. update example

[Prastiwar]

Hey, thank you very much for really nice pull request.
I changed convention to fit this project.
I fixed ReadOnly error (in Unity 2019) that is in Unity.Collections instead UnityEngine.Collections
I fixed #2