Improved Sub-Task section within the issue view

Currently there is no visual difference or marker that a user has selected to list only open sub-tasks (as it was before with the selected tabulator). Thus, if you forget to switch back to list all sub-tasks there might be the affect to the user that sub-tasks has disappeared.

Please add a visual flag for that filtered view! Eg. add '(only open)' beside the header 'Sub-Tasks')!