Eliminated the need to pass the owning BaseMetroTabControl to MetroTabItem
removed superfluous property OwningTabControl from MetroTabItem
updated migration guide
using TreeHelper.TryFindParent
removed obsolete c'tor
don't know where this using came statement from...
Merge pull request #1624 from thoemmi/MetroTabItem-without-OwningTabControl

Eliminated the need to pass the owning BaseMetroTabControl to MetroTabItem