Ignore disabled system shortcuts.
Ignore disabled system shortcuts.

This was implemented before in 0633545a, but lost during the rebase.

mpapp/MASShortcut,MacPaw/MASShortcut,MacPaw/MASShortcut,shpakovski/MASShortcut,shpakovski/MASShortcut,shpakovski/MASShortcut,ianyh/MASShortcut,baz/MASShortcut