Increase extensibility of BasicGraphicsScene and GraphicsView

[nolankramer]

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation/refactoring

Description

This is a general quality-of-life change that increases extensibility of the BasicGraphicsScene and GraphicsView classes.

The rationale behind this is to enable users to do a few things:

1. Users that wish to subclass BasicGraphicsScene should be able to override default event behavior. This is enabled by marking the on* methods as virtual.
2. This is similarly done for the GraphicsView class. Enabling users to customize behavior when subclassing.
3. A setNodeGeometry() function has been added to BasicGraphicsScene, such that users can customize the default AbstractNodeGeometry used by the BasicGraphicsScene.

Testing

• Qt version tested:
• Existing tests still pass
• Added tests for new functionality (if applicable)

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