ThumbView has incorrect position for some cases

[IlyaBaudin]

Bug description:
ThumbView has incorrect position for some cases.

Steps to reproduce:

1. Make an NSSwitch Outlet to NSViewController
2. Predefine default value for NSSwitch is ON
3. Call .setOn(on: true, animated: false)
4. In this case bgView color changed, but thumbView position will not changed and it always use OFF (left) position.

[IlyaBaudin]

I made a pull request. Changes should fix this issue.
@hpennington please take a look.

#3