Confirmation dialogs should support standard keyboard shortcuts

[njx]

There are a set of standard keyboard shortcuts on the Mac in various common dialogs, such as the "unsaved changes" dialog. I'm not sure what they all are, but the one I use frequently is Cmd-D to mean "Don't Save".

Windows has some common shortcuts/mnemonics too: e.g. y/n/Esc to dismiss Yes/No/Cancel dialogs. I always want to use 'n' for "Don't Save" -- that seems to be the button we all want quick access to the most :-)

We should figure out what's standard on both platforms and support them.