GUI for Stetl ETL Execution

Several requests for "a GUI for Stetl" were made. There are two GUIs to consider:
1. a "construction GUI" to create Stetl Chains, i.e. a Stetl config file
2. an "execution GUI" to execute the actual ETL (based on Stetl config file)

The first would require a tool "FME-like" to draw Inputs, Filters and Outputs and connect and paramterize them. The second is easier: manage the execution for a single Stetl config file. This issue adresses only the second/execution GUI.
## Requirements

An initial set of requirements:
- cross-platform (Win, Mac, Lin)
- easily (un)installable (installer)
- minimal scenario: choose ETL config, parameterize (e.g. inputs and output parameters), execute

Secondary requirements:
- prepare for becoming a Cloud service
## Implementation options
- cross-platform Python GUI framework like PyQT, WxPython etc
- Webtech cross-platform: Electron or lighter: PyWebView: https://github.com/r0x0r/pywebview
- Jupyter-based: https://github.com/r0x0r/pywebview ? (GUI via websockets)

The issue defines an execution GUI for choosing and parameterizing a Stetl config, then running the ETL, but names no source files, tests, or execution entry point. First review Stetl's existing config execution flow and decide between the listed cross-platform approaches; done requires an installable cross-platform GUI that supports the minimal scenario and leaves a path toward cloud execution.

Written by the indexing model from the issue text.