Add support for full span highlighting to modal and use it in the "Add Space View or Container" modal

[abey79]

What

This PR does the following:

• Make sure the modal's clip rect is the same size as the modal itself.
• Add a full_span_separator() helper function that's similar to ui.separator() but draws a full-span line and doesn't add vertical spacing.
• Add a "full span" mode to our modal. In this mode, the vertical spacing is entirely left up to the user code.
• Use the above in the "Add Space View or Container" modal for a nice, full span hover highlighting.
• Add an example in re_ui_example showcasing how full span modal and ListItem work together.
• Fixes Improve modal window styling to support full span highlighting #4673

re_ui_example with ListItem:

Add SV or Container:

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

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