Restore ability to bind dead keys on macOS

A previous fix for IME input bails out of keystroke handling if the
legacy keyCode property is 229. Unfortunately, this is also the value
of keyCode for keystrokes like alt-u, which don’t pop up the IME menu.
These keystrokes should be handled normally. It seems like the best way
to distinguish these keys from the IME case is that their key value is
‘Dead’.

Author: Nathan Sobo

src/keymap-manager.coffee

@@ -492,7 +492,7 @@ class KeymapManager
     # 229, which is the "composition key code". This API is deprecated, but this
     # is the most simple and reliable way we found to ignore keystrokes that are
     # part of IME compositions.
-    if event.keyCode is 229
+    if event.keyCode is 229 and event.key isnt 'Dead'
       return
 
     # keystroke is the atom keybind syntax, e.g. 'ctrl-a'