Support exclusion of elements from focusgroups

[echo-vladimir]

The focusgroup=none feature allows elements and their descendants to opt-out from focusgroup participation. This prevents focusable elements within a subtree marked with focusgroup=none from being part of the parent focusgroup, ensuring they are skipped in the focus navigation cycle and do not respond to arrow key navigation. The focusgroup=none attribute enables more granular control over focus behavior, especially in complex components like accordions or nested structures, where certain elements need to be excluded from the focus cycle.