Expand/collapse widget areas based on whether they have content

[critterverse]

This was originally part of #31959 but splitting it off into its own issue as a potential improvement for the Appearance widget screen:

It’s slightly confusing that the widget areas that are in use are closed when you first enter the screen, and I imagine some users might think that their content disappeared. Could we have empty areas default to closed and areas with items default to open/expanded? (Inactive widgets, if that section remains, could default to closed)

[noisysocks]

Why not open all areas, including empty ones, but excluding Inactive widgets, by default? Agree that having some closed by default makes it difficult to drag and drop.