Create RPGCore.DataEditor to edit and maintain game data #28

Features:

• Basic data editing of lists, dictionaries, objects, and values
• Support for editing data structures as defined in C#
• Saving and loading data to and from files
• Support for editing comments
• Support for abstract and unknown type editing
• Support replacing value types with an alternate type
• Enum data types (serialized as both string and number)
• Support change events and model binding
• Support for generic types
• Support for data variants
• Support for polymorphic types
• Undo/Redo history
• Support resetting values to their default state
• Support data variants inside a larger container (e.g. prefabs in the context of a scene)
• Unique identifiers for values in hierarchy (URLs)
• Move to a Command/Procedure API for data editing
• Networked data editing