This fixes ticket #100, and is best explained using a little example.
Consider the following layout:
+---+---+
| | X |
+---+---+
| X |
+---+---+
Where X marks a window, so you have an empty container in the upper
left, the container on the bottom is snapped to the right. Before
this commit, nothing would happen when focusing "above". After
this commit, the upper window gets focused.
