[Sprite] improve "Tree" view in inspector

[janvrany]

This commit improves "Tree" view in inspector in several ways:

• sequence of ELets is "flattened" and shown is list instead of deeply nested tree
• it shows multiple columns, most notably "Class" and "Bind" with bound id (or nothing if expression does not bind anything)
• fixes few DNUs when diving into annotations

I personally find this easier to work with for more complicated, deeply nested programs.