Console outputs a warning about keyWindow call at launch

[1024]

Console out puts the following message at app launch.

KVO autonotifying only supports -set<Key>: methods that return void. Autonotifying will not be done for invocations of -[NSApplication _setKeyWindow:].

However, the app works well.

This is caused by binding to keyWindow on MainMenu.xib.

[1024]

Add a method that returns keyWindow to avoid console warning arise from binding (fix #23)

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