aggregateMenu: Include action buttons in width computation again

Commit 1b169655 removed the system indicator from the list of children that are considered for the overall menu width, because we do want the log-out submenu to adapt to the available width.

However as a side effect, action buttons no longer contribute to the width either, so if extensions add additional buttons, the menu is likely to overflow.

Avoid this by only adding the button group to the list of size children.

Fixes #1094 (closed)