Change default command mapping for Mac

[jbenhamou]

The default command mapping is already taken in Mac using the Mac OS X 10.5+ keymap.

It's mapped to Refactor -> Extract -> Type Alias and also Plug-ins -> Kotlin -> Type Alias

It took me a while to figure out whey this wasn't working. It will probably be a better out-of-the-box experience (I used Shift-Alt-Cmd-,).