Add tree widget

[hydra]

First attempt add adding a dynamic tree widget.

Features:

• nodes are widgets.
• nodes can be expanded/collapsed.
• tree can be accessed and manipulated from within the node widgets, e.g. by 'Button::on_click'.

TODO

• Make performant.
• Use appropriate expand/collapse icons instead of > and v.
• Only show the expand/collapse icons when there are children.
• Remove use of the .contain() which is currently there for visualization/debugging purposes.
• Document.
• Cleanup code.
• Squash or cleanup commits.

Issues:

• the performance is currently abysmal once you add around 4 nested children. adding another child using the 'Add child' makes the UI stall for longer and longer periods for each nested child, scrolling becomes very slow.

Notes:

• No idea if the approach I took, of using widget composition, was a good idea.
• Probably some unneeded code, as I tried various approaches that didn't work until I stumbled on one that does.
• Probably there's some unnecessary cloning going on too, maybe on the whole tree itself including all the nodes. Clone for Tree has been removed.

[hydra]

rebased

[ecton]

I'm not sure what specific questions there are for me to address on this PR. At this time this control is not something that I personally have a desire for in Cushy so I am not going to be working to finish this myself anytime soon. I will answer questions if you have them.