Despite its name, IsItemPresent() doesn't return a boolean, it returns an index.  Made this more clear where it's used.

Introduced a similar method named IndexOfFirstOccurence() that returns the index, as you'd guess from its name.  Changed many uses of IsItemPresent to IndexOfFirstOccurence().

Changed HasProp() and HasImage() from returning int to returning vtkTypeBool. Also, with VTK_LEGACY_REMOVE, changed behaviour to actually return 0 or 1, instead of returning a 1-based index.