Expand/Collapse TreeViewItems with arrow keys

[robkri]

It would be really nice if one could browse a TreeView by only using the arrow keys. Therefore it would be needed to expand and collapse an item with the right (expand) and left (collapse) arrow key. This is also the behavior what windows explorer offers.

Although moving vertically through the tree by using the up and down keys already works, it looks a bit odd that the black frame is larger than the the actual item, thus showing a white line between the black frame and the grey background when the item is hovered:

This request probably is connected to this bug: #493