Add up/down arrow control to NME search results

[PatrickRyanMS]

When using the new "spacebar to search" functionality, with each key press the results query updates to any nodes that match the input term. A user may see the node they want in a position other than the first. For example, typing "col" will return Color3 first and Color4 second. If the user wants a Color4, they need to continue typing the rest of "color4" to get Color4 return first to press enter and place the node. The other way is to grab the mouse and hover Color4 and click. This feature would speed up the process. Consider the following flow:

1. The user presses spacebar and types "col"
2. The results show Color3 first, highlighted the same as if it were hovered with the mouse. It shows Color4 as the second return.
3. Since the Color3 is highlighted, pressing enter will place the Color3 node.
4. If the user wants to place a Color4 instead, pressing the down arrow once will move the highlight down one space in the returned list to highlight the Color4 node.
5. With the Color4 node highlighted, pressing enter will place the Color4 node
6. If the user had the Color4 node highlighted, but decided instead to place a Color3 node, pressing the up arrow once will move the highlight up one space in the returned list to highlight the Color3 node.
7. Repeated presses of the up or down arrow will move the highlight up or down by one item with each press.

For this to work as we have the initial feature working, we always need to highlight the first node in the returned list so that the user knows that pressing enter without using the mouse will return the first node. Currently we don't highlight anything until the mouse hovers a returned node in the list. Using the arrow keys to modify what is selected by moving the highlight on arrow press will often result in fewer key presses and no lost time reaching for the mouse.