Move Roundness to libcosmic

[git-f0x]

• I have disclosed use of any AI generated code in my commit messages.
  • If you are using an LLM, and do not fully understand the changes it is making to the code base, do not create a PR.
  • In our experience, AI generated code often results in overly complex code that lacks enough context for a proper fix or feature inclusion. This results in considerably longer code reviews. Due to this, AI authored or partially authored PRs may be closed without comment.
• I understand these changes in full and will be able to respond to review comments.
• My change is accurately described in the commit message.
• My contribution is tested and working as described.
• I have read the Developer Certificate of Origin and certify my contribution under its conditions.

Moves Roundness from COSMIC Settings to libcosmic and merges the density.rs file into spacing.rs.

Also updates dependencies, and moves some shared deps into the workspace.

Should I add the rustfmt config, or would it be better to wait for theme v2 to avoid conflicts?
Edit: I've added it, since the theme v2 branch doesn't seem to touch imports much, so conflicts should be minimal (or none).