Add feature to manually expand/collapse the tree

[ChristophPopp]

• Add e.g. small "+"-button in every node to manually expand its child or parent nodes if available.
• If they are already expanded, turn the "+"-button to a "-"-button and provide the opportunity to collapse the child or parent nodes again.
• Hovering over the buttons could give a hint on the available action
• This image shows an example how it could look like: