[GUI] Create outcome editor panel for extensive forms

[tturocy]

This changes the UX for creating and editing outcomes in the extensive form.

Previously, we attempted to edit payoffs in the extensive form by popping up a text control more or less in the "right" place on the extensive form canvas, and allowing edits via e.g. tab traversal. This had a number of poor features:

• It was quite brittle and doesn't work very well with having to check for and validate "unsaved changes" in an edit.
• It did not expose outcome labels - which we now consider to be important (in fact essential)
• Even when editing only one payoff, it still makes sense to be thinking about the outcome as being associated with a vector of payoffs.

This replaces the editing mechanism with a popup window which displays the outcome label and all payoffs. Edits are accepted by Enter or clicking elsewhere in the parent window, and are dismissed via Esc, so changes to the outcome are atomic at the edit level (rather than the individual payoffs level as before).

[tturocy]

All, this is also still WIP but testing and suggestions are very welcomed.

[edwardchalstrey1]

Working for me, though I wonder if there's anything you can do to make it clearer that double click lets you edit the outcomes. Also, it then made me think "why can't I double click to edit action labels"

[d-kad]

looks good to me -- one small thing to flag, likely outside this PR's scope: payoff input doesn't normalize negative zero