Fixes #5594: Vertical button groups show use max-width, not width, and be scoped to immediate children buttons only

* IE8 refused a negative z-index to place the previous :after behind the .arrow, so I had to flip the styles.
* .arrow is the gray border/background, .arrow:after is now the white inner arrow
* Added a hex IE8 fallback to ensure the arrow shows for <IE9 as rgba() isn't supported
* Simplified some of the CSS for the popover carets

Previously, we were using a generic label selector, but as this was changed in the main forms awhile back, we need to change it here.

Fixes #5471: Remove color from .navbar and .navbar-inverse and reassign to .navbar-text to prevent unintended cascading

* When :empty, display: none;
* Add documentation for change in behavior