[UX] Layouts: Add a "remove" link/button to the block configuration dialog

[klonos]

We did not reach consensus in #1725 and that ticket was closed, with the work moved into a contrib module: https://github.com/backdrop-contrib/block_remove

Since the UX WTF from the scenario from that issue is still a thing, I thought that we could perhaps compromise by adding an option to allow people to remove the block without having to close the dialog. So instead of this:

1. Hover over a block in the frontend and click on the contextual links cog.
2. Click on the "configure block" contextual link and be brought to the respective layout edit form with the "Configure block" dialog for that block already open for them.
3. Wait! ...there's no "remove" button available -> Close the dialog.
4. Scroll to the part of the layout form that holds the block ( [UX] Add # anchors to blocks in the Layout UI. #1724 )...
5. Click the down arrow on the "configure" dropbutton.
6. Click the "Remove" option.
7. Save changes -> back to the front-end with the block removed.

We could have less steps (while keeping things happening in the backend - since that was one of the main objections in #1725). This is how envision this could work:

1. Hover over a block in the frontend and click on the contextual links cog.
2. Click on the "configure block" contextual link and be brought to the respective layout edit form with the "Configure block" dialog for that block already open for them.
3. Click the newly-introduced "Remove block" button that'll be available.
4. Confirm.
5. Save changes -> get redirected back to the frontend with the block removed.

If we made the confirmation step be "Confirm and save layout" instead, then we'd end up with 1 additional step less.

[olafgrabienski]

@klonos What will be the intended behavior, if a block is placed in several layouts? Only remove the block on the one layout, or handle other layouts as well? (The contrib module provides an "option to select which layouts to remove the block from".)

[klonos]

I believe that the default should be to remove it from the current layout. I wouldn't mind if there was a message to point out that the block is being used elsewhere, or to offer removing it from other layouts as well.

The intention here (as with most UX improvements) is to save users clicks, and in general to make working in the admin UI less frustrating or tedious.