Add support for active_tab

[jleibs]

What

Referring to an object created in the same constructor is somewhat awkward.

Forcing users to create the object out-of-line so they can assign it to a variable and then refer to it is just a pain.
Specifying a name or index seems like a convenient middle ground.

Example

            Tabs(
                BarChartView(name="Bar Chart", origin="/bar_chart", contents=["/bar_chart/**"]),
                TimeSeriesView(name="Curves", origin="/curves", contents=["/curves/**"]),
                TimeSeriesView(name="Trig", origin="/trig", contents=["/trig/**"]),
                TimeSeriesView(name="Classification", origin="/classification", contents=["/classification/**"]),
                active_tab="Curves",
            ),

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
• If applicable, add a new check to the release checklist!

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[emilk]

Nice