[V4 + V5] Add Separator For Same-Colored Button Groups

Created by: einfallstoll

I think it's incredibly hard to visually distinguish between regular buttons with a caret (which open a dropdown) and buttons in a button group with a separate button for the dropdown.

Regular buttons with a caret that opens a dropdown:

Button groups with a separate caret button that opens a dropdown:

To see the difference between the buttons, you either have to get a feeling for margins so you can guess the bigger margin means a separate button, or you have to hover the button:

I think this not only applies to button groups for dropdowns but also to regular button groups. I also got critical responses for not visually separate buttons:

The same applies to V5 as well.

I'm quite sure there was a separator in the past, which got removed at some point. So I request to re-add the separator again, as it makes it easier to distinguish between same-colored button groups as well as making it more accessible for visually impaired people.