ui.tree select/deselect, tick/untick programmatically

[arunaseva]

Link to the description:

#3436 (reply in thread)

[falkoschindler]

Thanks for the pull request, @arunaseva!

I just reviewed your code and did some minor changes:

• I changed select(None) to deselect the nodes instead of doing nothing. This feels more intuitive.
• The code for untick() could be simplified a bit. We don't need to intersect before computing the difference.
• The tests could be simplified with a label showing the current selection and tick list, resulting in rather readable should_contain statements.
• Last but not least, I decided to split the select and tick demo. This reduces complexity and is more in line with the existing expand/collapse demo.

Let's merge into main! 🚀