feat(ui): Added themeable stream icons

[renefloor]

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Description of the pull request

This adds the stream icons as font icons, so we can use the more efficient IconData instead of SVG's.
This also add an option for customers to override (some of) the icons as they are supplied in a theme.

The icons themselves are currently pretty broken, but this should be improved by better svg exports. This PR adds a good structure which we can use going forward. Another PR can fix the actual icons themselves.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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[renefloor]

I think ThemeGen would be a bit overkill here, without it we can keep the class much more simple.