Hide all scratchpad windows

There is the `--scratchpad recover` special action.
I wonder if it'd be worth adding sth like `hide_all` that does the opposite: Toggle-off any scratchpad windows around to "clear the view" when multiple scratchpads have accumulated.

Start at the existing `--scratchpad recover` special action and trace how scratchpad windows are selected and toggled. Add the opposite action so every accumulated scratchpad window is hidden, then verify that invoking it clears the view when multiple scratchpads are present.

Written by the indexing model from the issue text.