Draft: Add a "Minimalist" GUI

This MR adds a proof-of-concept "minimalist" GUI written in pyqt5: it allows to control a Constellation with a minimum of buttons (and feedback for that matter).

This could e.g. be extended to act as a main tab in a more feature-complete GUI or as a simple operators (touch) interface.

While this could be merged as-is, I would like to add a couple more features:

• pre-fill the run identifier and sequence trailer from the Constellation (similarly to what @simonspa's GUI does)
• update the run id sequence number at an appropriate place (not the start of the run)
• move to pyQt6
• add some hint of it to the documentation (somewhere)

Otherwise, the GUI is essentially functional.

Further ideas for improvement:

• test whether some actions, such as Start should be run in separate threads
• provide more user feedback either in a dedicated small field (best) or the status line
• add a button/tab to view the loaded config file
• make it prettier!
• test in practice! 😸