feat: docks menu, not closing docks by clicking on X, main figure unclosable

Playing with docks yesterday, I think usability would be improved by adding the following features:

• a "Docks" menu, to see a list of those ; clicking on it make them appearing or raising to top-level ("ensure visible")
• when clicking on the red cross to close a dock, let's hide it and disconnect it(?) but not destroy it
  • like this users can retrieve a closed dock from the menu
    • if the dock widgets were disconnected, refresh them and connect them again
  • the "Docks" menu could have an option to close and destroy a dock

If the 2 above are implemented, I think it's fine. Still there is one other possible feature to simplify users life:

• make the main figure unclosable
  • it seems there is nothing to put it back if closed by mistake ? It is not a waveform, not an image and it does the auto-update thing ?