Behaviorally: This change makes it so that Enter will never cancel
a dialog, unless that dialog only contains info to be read. Escape
will never unpredictably be treated as Enter. And some dialogs without
Escape to cancel, now can be canceled with Escape.
A lot of this just refactors dialogs to define the cancel button
at the top, not as a hotkey on a cancel button deep in the XML.
I think this makes enter/escape behavior in dialogxml files more
legible at a glance.
Better editing for talk node keys
* The fields start out empty, and it is *allowed* to keep *one of them* empty. (A node only needs one key to be useful, right?
* When the node is saved, empty strings are replaced with `" "`. If this is not harmless to the game logic, then I'll need to make the game aware that either key might be `" "`.
* You cannot type more than 4 characters in these fields (unless there's a way to insert characters I'm not aware of).
* If you put 'buy' in a field, it automatically becomes 'purc'.
* There is a note next to the response keys (kind of squished in the corner) enumerating the special logic of the in-game Buy and Sell buttons.
