Add a simplified new function to help making filters.

This should allow writers to add filters more easily

filter = Filter.new(svg, 'GuasianBlur', stdDeviation='1 1')

This filter is now ready to use in an element and the url formatter can be used:

item.set('filter', filter.get_id(as_url=2))

We're currently not tracking filter ids as we are mask and clip-path ids, so setting the id will not update the filter using elements.

What does the merge request do?

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Implementation notes

Summary for release notes

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Checklist

• Add unit tests (if applicable)
• Changes to inkex/ are well documented
• Clean merge request history