Add a theming API for visualizations

[s9ferech]

Pull Request Description

This solves issue #1337.

Important Notes

Checklist

Please include the following checklist in your PR:

• The CHANGELOG.md was updated with the changes introduced in this PR.
• The documentation has been updated if necessary.
• All code conforms to the Rust style guide.
• All code has automatic tests where possible.
• All code has been profiled where possible.
• All code has been manually tested in the IDE.
• All code has been manually tested in the "debug/interface" scene.
• All code has been manually tested by the PR owner against our test scenarios.
• All code has been manually tested by at least one reviewer against our test scenarios.

[s9ferech]

I still have two questions:

1. Should I update the documentation as part of this PR?
2. Do we need to reload the visualisations on style changes?

[radeusgd]

I still have two questions:

1. Should I update the documentation as part of this PR?
2. Do we need to reload the visualisations on style changes?

I'm not sure about (1) (I guess @wdanilo or @farmaazon will have a say here).

As for (2), I think it would make sense - if the colors for types can change with the theme, the visualization also needs to reflect this change to be consistent. Maybe we could allow some kind of callback telling the visualization to refresh its color scheme, but I think that would make it more complex for visualization creators, so IMO refreshing is the simplest solution here. What do you think?

[wdanilo]

1. Of course. Docs are always part of the PR.
2. Not now. We do not have proper theme reloading now, this will be a separate issue.