Add multiple panes for Middle Wide Layout

[Ked-Ki]

Previously, the Middle Wide Layout did not respond to the increase/decrease main pane count shortcuts. Now it does, so if the screen currently looks like this:

+-----+---------------+-----+
|     |               |     |
|  1  |               |  3  |
|     |               |     |
+-----+       0       +-----+
|     |               |     |
|  2  |               |  4  |
|     |               |     |
+-----+---------------+-----+

You can then push the "increase main pane count" shortcut, and it changes to this:

+-----+---------------+-----+
|     |               |     |
|  2  |       0       |     |
|     |               |     |
+-----+---------------+  4  |
|     |               |     |
|  3  |       1       |     |
|     |               |     |
+-----+---------------+-----+

[ianyh]

Cool!