Improve browser back-button navigation

[emilk]

What

Loading an example and pressing back will keep the example loaded, but still get you back to the welcome screen.

Loading the example again will clear the old first before continuing.

You can open examples in a background tab by holding down a modifier or middle-clicking.

And fixed small annoyance that required an extra "Back" press to get back to the original source.

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: rerun.io/viewer
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[jprochazk]

Can we also make this work with mouse 4 and mouse 5?

[emilk]

@jprochazk Sure, try out #5792 (I don't have a mouse)