Come up with a plan for the scene graph enhancements

The first goal is to be able to construct a Q3DSUipPresentation, including the scene and slide graphs, programmatically.

There are at least three big steps involved:

1. Make all "properties" settable. (and massage Q3DSPropertyChange(List) so that notifyPropertyChanges can be used with such changes as well)

2. Make set/add/etc. kind of functions to build animation tracks, assign objects to slides, etc. Many of these will need to be notifiable too.

3. Graph change notifications. The "properties" are handled in #1 but the graph itself needs NodeAdded, NodeRemoved, etc. type of dirty flags and notification mechanism.

(4. What to do with Q3DSEffect and Q3DSCustomMaterial?)

Then comes the even bigger task: make Q3DSSceneManager and friends behave when the graph or anything changes on the fly.

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