[HACK] [WHAT] [NO] Fix Mac command-key shortcuts in modal dialogs

This terrible, horrible no-good pull request fixes Mac command-key shortcuts (like cmd-D to dismiss the save dialog) in modal dialogs. The problem arises from the operating system capturing keyboard events that correspond to shortcuts for menu commands. The shell handles these keyboard events and translates them into command names,  which are translated into commands and executed at the JavaScript level. This hack intercepts these command names and, when a modal dialog is open, forwards them as `shellCommand` events which are handled by open modal dialogs. The `shellCommand` events are then translated into their corresponding key bindings (e.g., `edit.duplicate` --> `Cmd-D`), which are used to manufacture a corresponding `keydown` event, and which are immediately handed off to the modal dialog's `keydown` handler. 

(You may wonder why the command name forwarded from the shell isn't immediately translated into a keyboard event. To do so requires importing `KeyBindingManager` into `ShellAPI`, which seems to cause a circular dependency that makes Brackets sad.)

So, yes, this is very ugly, but, hey,