Add property groups to type definitions

When a generic CMIS client should display the properties of an object, it can only show them as a flat list in a random order. That can be confusing to end-users. A simple example is an address with properties for the street, the city, the state, and the zip code. It would be nice to group them together in a meaningful order.

The type definition should contain an (optional) ordered list of property group definitions. A property group consists of a display name and an ordered subset of the properties of that type. A client could than iterate through the property groups and within the property groups through the properties and display them. Not all properties need to part of a property group.

Property groups only exist on the type definition level. They don't have any effect on the way the properties a transmitted.