support ChildrenExist prop in schema tree

[adameat]

1. we should never expand a tree element and display "No data" for a child with no children - just reset its state to non-openable.
2. for every children there is could be a property called "ChildrenExist"

and I suggest the following logic:

a) if it doesn't exist - work like before (like in n.1)
b) if it does exist and false - disable opening the tree element
c) if it does exist and true - show toggle to allow opening it (but ready to fall back to n.1)

[antonkovalenko]

In versions without grandchildren nodata sign might be removed
> and loader must remain as they are