@jsgf suggested that it would be nice to have a degree of padding
around the terminal cells. This commit adds some plumbing for this;
```
[window_padding]
left = 10
top = 0
right = 10
bottom = 0
```
The left and top padding are used top compute the top-left coordinates
of the terminal cells. The right and bottom padding act as minimum
values; the actual padding used may be larger depending on the size
of the window and the number of cells that fit the available space.
top padding > 0 and the tab bar needs some work.
