Improve the placement of watched output.

According to the basic rules: 
With 2 panes, always put the compiled JS in the _other_ pane.
With more than 2 panes, put the compiled JS to the right (or below), unless there is no pane to the right (or below).

More detailed rules:
With the following **Layouts**, with the following **Input** panes selected, **Output** will appear in...

Layout | Input | Output
-------- | -------- | ---------
1x1 | Only pane | New column to the right
2x1 | Left | Right
2x1 | Right | Left
1x2 | Top | Bottom
1x2 | Bottom | Top
3x1 | Left | Middle
3x1 | Middle | Right
3x1 | Right | Middle
4x1 | Left | Mid-Left
4x1 | Mid-Left | Mid-Right
4x1 | Mid-Right | Right
4x1 | Right | Mid-Right
2x2 | Top-Left | Top-Right
2x2 | Top-Right | Top-Left
2x2 | Bottom-Left | Bottom-Right
2x2 | Bottom-Right | Bottom-Left

Closes #182