Graph Editor: Add non-editable Tokens

[kwokcb]

Scope

This is part 2 of adding in token support:

1. Show up like inputs do, but in a new "Tokens" section. Tokens should show up similarly to how string inputs show up, except the strings are non-editable inputs.
2. Show tokens from definition if they exist. Initially all tokens on the instance and nodedef can show up. This applies to nodes and nodegraphs.
3. Scope: Tokens on children of the element being examined should not show up. e.g. you don't want tokens for nodes in a nodegraph to show up.

[jstone-lucasfilm]

Now addressed by @xipaja in Dev Days contribution #2605!