Update label constraints when textContainer.lineFragmentPadding changes

[markterluun]

I noticed that the constraints were only updated after updating the textContainer, not after updating textContainer.lineFragmentPadding and decided to make a fix for it.

[rtemple6]

+1 on these changes.

When trying to initialize new instance of KMPlaceholderTextView w/o lineFragmentPaddingObserver, the placeholder inset does not update.

private lazy var messageTextView: KMPlaceholderTextView = {
   let view = KMPlaceholderTextView()
   view.textContainer.lineFragmentPadding = 0
   return view
}()

Before

After