Focus: Strategy

[Nemikolh]

With PR #12 focus is now using a cursor strategy to find the choose the next node when changing the focus. It works well on the shop_inventory example.
However, we can still build example that might be counter intuitive when switching the focus, consider the following case:

+------+   +------+
|  A   |   |  B   |
+------+   +------+
           +------+
           |  C   |
           +------+
+------+
|  D   |
+------+

Assuming you start at D with the cursor centered on D, if you do a focus_up, you'll end up on C. And if you do again focus_up, you'll end up on A.