Updated HighlightAdorner for Expanded items.
Reverted rect.IsEmpty and instead use tvItem's height

Realized that using tvItem.IsExpanded without changing how the rect was constructed would introduce regression bugs.  After about a quick bit of research found that the TreeViewItem has an ActualHeight property that stays accurate upon expanding/collapsing.
Got rid of unneeded variable.

Since I changed it from the gridrow's height to the TreeViewItem's ActualHeight..
Merge pull request #99 from claudekennilol/master

Updated HighlightAdorner to highlight the entire expanded item.