Fix tm_variable_browser: report plot (#941)

# Pull Request

Fixes #936

Generates code inside the qenv for it to be reproducible and make the
module functional.

I tried to make minimal changes but at the same time to make it fast to
render by adding new `req()` to invalidate reactives.

Note that the code created for the plot has some changes as it has the
construct for the new S7 labels. Maybe there are other issues with it.

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Author: llrs-roche