Highlight non-selected minibuffer prompt

[Juri Linkov]

Only today I realized how to address the long-standing complaint
that sometimes accidentally switching from the minibuffer (that
e.g. asks a yes/no question) remains unnoticed that leaves
the minibuffer active for a long time.

The solution was already invented long ago by minibuffer-depth-indicate-mode
that highlights the recursive minibuffer with the minibuffer-depth-indicator 
face.

The same solution could be used to add a highlighting to the minibuffer prompt
after switching from the minibuffer that still remains active,
thus turning the attention to a possible problem.

I still have no idea where to implement this feature.  Maybe in the same
package mb-depth.el?  Then should it be a separate mode or better to handle
by a new option in the same minibuffer-depth-indicate-mode?
Should it reuse the same minibuffer-depth-indicator face
or add a new one?