Remove a private API in PSMTabBarCell

Directly assigning to _controlView of a NSActionCell is private and not allowed by Mac App Store.
An _customControlView is used to replace _controlView in PSMTabBarCell to avoid direct assigning to _controlView.
This should be the last private API to remove before going to Mac App Store.
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Merge pull request #2030 from origsun/master

Remove a private API in PSMTabBarCell