Kino.Tree: auto-expand tuples of up to 6 elements

[stefanchrobot]

Closes #387.

Auto-expand tuples of up to 6 elements in Kino.Tree by default.

I've introduced the following changes:

• Add "kind" (e.g. "tuple", "list", etc.) attribute to tree's data,
• Use the "kind" attribute to change the logic for auto-expanding data; this now makes it easy to apply the same logic to lists and maps (if desired),
• Flatten the structure of the tree data: introduce "expanded_before" and "expanded_after" attributes,
• Refactor the UI code: rename expanded and setExpanded to isExpanded and setIsExpanded to better fit JavaScript conventions,
• Refactor the backend code: introduce leaf_node and branch_node to ensure that we build consistent data (without introducing more structs/types).

[jonatanklosko]

Perfect, thank you!