Cleanup the extensions menu, deliver on ux#140

Jonathan Neuhauserrequested to merge
joneuhauser/extensions:renaming into master

What does the merge request do?

Fixes #385 (closed)

Fixes ux#140 (closed)

Implementation notes

Also title-cased all extension names. Extension IDs (and file names) were only changed where absolutely necessary.

Summary for release notes

The extensions menu has been cleaned up: Extensions have been renamed to more representative names and the grouping is more logical.

Checklist

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

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