Enable realtime rate report in melds

[SeanCurtis-TRI]

• Turn on the panel
• Compute realtime rate for only one _ViewerApplet at a time

Resolves #17661

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This change is 

[SeanCurtis-TRI]

+(type: feature request)

To see the effect execute the hardware_sim examples:

$ cd drake
$ bazel run //tools:meldis -- -w &
$ bazel run //examples/hardware_sim:demo_cc

You should see a rtr window in the upper left corner.