Move update of graph theme to Graph.__call__

[antonymilne]

Description

As prototyped in #166, a bit of further refactoring with no change in functionality.

Before: the code to update graph theme lived in _callback_mapping_utils.py.
Now: it lives in Graph, plus a couple of other improvements.

Why is this (probably) better?

• callbacks no longer need to all carry around ctd_theme
• callbacks are not directly tied to Graph implementation
• the duplicated logic between the update_graph_theme callback and the update_layout call is centralised to a single Graph._update_theme method
• the update_theme callback is generic so in theory works for components other than Graph so long as they define a _update_theme method (probably not very useful in practice)

Possible drawbacks:

• Graph.__call__ is coupled to the Dash callback context. This was actually aways my original intention, but it was never made explicit before, and the current actions implementation works somewhat differently. To be seen whether it causes problems

Future work:

• (small/easy) see if we can get rid of the _update_theme call in Graph.__call__ altogether by making update_theme a clientside callback. This would be even nicer, but the current solution is perfectly performant so it's not a big problem
• (bigger/difficult) consider whether more functionality should move from callbacks into __call__

Checklist

• I have not referenced individuals, products or companies in any commits, directly or indirectly
• I have not added data or restricted code in any commits, directly or indirectly
• I have updated the docstring of any public function/class/model changed
• I have added tests to cover my changes (if applicable)

Types of changes

• Docs/refactoring (non-breaking change which improves codebase)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Notice

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[petar-qb]

Great tidying up! 🚀