Cmd-D doesn't select Don't Save on Mac

[njx]

1. Edit a file
2. Close it without saving
3. Hit Cmd-D in the confirmation dialog

Result: Nothing happens.
Expected result: Should select "Don't Save".

[njx]

This has probably been broken since we transitioned to native menus--the issue is that Cmd-D is a shortcut that's listed in the menu, so we never get the keyboard event for it. I think this is low priority since Cmd-D is no longer standard for "don't save" in save dialogs on Mac post-Lion (it means "switch to desktop folder" now, I think).

[njx]

Reviewed. To @gruehle to figure out what we could do here.

[njx]

Decided to move this to backlog as part of a larger story around more flexible key event handling. https://trello.com/c/I07x1pO0