Expose vgui::Label::SetCenterWrap(bool) to the lua

[IVogel]

As far as I can see, Garry's Mod should have this function in the Label class, since it has it in vgui::TextImage.
Label also uses TextImage for text rendering.

If m_bWrapCenter is true, then RecalculateCenterWrapIndents should be called and it should update all horizontal offsets in the m_LineXIndent array.

Also, Source SDK 2013 headers have void SetCenterWrap( bool bWrap ); inside the Label class.

[robotboy655]

Related: #680