Eliminated the need to pass the owning BaseMetroTabControl to MetroTabItem

Currently `MetroTabItem` has a public property `OwningTabControl` which is required when closing a tab (confer #555 and #631).

I eliminated the need for this property by finding the `BaseMetroTabControl` at runtime. This makes it much easier to add tabs dynamically in a MVVM application, because you don't need to pass the `BaseMetroTabControl` to `MetroTabItem`'s constructor.

I kept the old, now superfluous constructor, but added a `ObsoleteAttribute`, because it is a breaking change. Feel free to remove that c'tor.
I noticed `TreeHelper` but associated it with the `TreeView` control like other XxxHelper classes. 

I'll fix my code by calling `TreeHelper.TryFindParent`