Add setters and getters for nodes and edges

[tonilastre]

Nodes and edges should have setters and getters for properties, data, and positions in the following manner:

getX()
patchX({ ... })
patchX((x) => ({ ... }))
setX({ ... })
setX((x) => ({ ... }))

The graph model should be able to listen for those changes and know if something has changed, which can be used to call renderer.render, a throttled call.

Another part of this task is states which is partially done here: #71