Fix a couple of row issues

[grokys]

#136 introduced a couple of issues, fixed by this PR:

• TreeDataGridRow.RowIndex was being incorrectly updated when rows were inserted while scrolled, causing such problems as:
  • Collapsing tree items using the keyboard causes overlapped items to be rendered #205
  • Selecting one row could cause multiple rows to be selected
• The unrealized focused element was not being considered when getting a realized element causing The selected row is duplicated in the table #200

Fixes #200
Fixes #205