Is your feature request related to a problem? Please describe.
When developing huge blocks that need a lot of configuration, you end up quickly with a lot of fields to manage (fields that are in the most cases wrapped inside Panel).
If the Panel aren't collapsed, it can take a "while" to find the input you want inside huge forms.

The props initialOpen can be use when you are coding your own blocks to suit your needs, but you also can use block librairies and you wouldn't have this possible option.

Describe the solution you'd like
I prefere when all panels are collapse at page load. (And some people don't, of course!).

I think one of the best solutions would be to add an action with Ctrl + Click on the Panel header to collapse all blocks at the same time.
This can allow a quick navigation inside the sidebar, for both use cases/preferences.

For me this would be a great feature, and maybe also a great feature for the people who get collapsed blocks when they don't want to!

(Thx guys, Gutenberg is great)