[UX] do not lock the admin bar when a modal is active.

As we have it currently, when there's a modal open ("Add block" dialog in the Layout admin UI for example), the only thing that one can do with the background is scroll up/down. I don't know if that was intentional or not, but it was proven helpful quite a few times in the past for me in cases where accomplishing a task in the modal required me to see what is in the back of it. So, please do not disable that.

The other thing is that we lock the admin bar menu and treat it as if it is part of the background. I had numerous cases where I needed to navigate to another page in order to fill in information in the modal. In such cases, I resort in opening another tab, navigating to the site in question and then using the admin bar in the new tab to navigate to the admin section I was looking for. Now, if it were that the admin bar was not locked, I would have simply used the admin menu and would have right-clicked -> open in new tab on the link I needed.

The alternative is to close the dialog, right-click -> open in new tab on the link that you need and then re-open the dialog. This is really annoying though and might be ok if you realized that you need to do it right after the dialog opens. If you have actually filled in something in the dialog already, then :angry: ...pisses me off.

Not sure what the implications would be on mobile mode, but there is nothing I can think of right now. If not on by default, then can we at least have a "Keep active when a modal dialog is open." setting under /admin/config/administration/admin-bar?