Change the way items are created #5319

In current G6 5.0, if a developer wants to customize a item, (e.g. node), he needs to know the basic composition of the node, like keyShape iconShape.

Now we think of item as a compound G graph instead of a set of atom shapes, developers are free to customize the node, only need to meet the minimum convention protocol.

For example, in a custom node, the node only needs to provide a public method to get the connecting ports.

More specific details will be added later.