Resize Taffybar window when screen config changes.

I occasionally switch my laptop between docked and undocked mode with the docked mode being hooked up to two monitors. Unfortunately Taffybar doesn't resize when going from a large screen to a small and vice versa. With these changes Taffybar will react to changes in the monitor setup of the screen and resize itself accordingly to span the full width of its configured monitor.

Please note that I replaced the
```haskell
_ <- on box sizeRequest $ return (Requisition w (barHeight cfg))
```
line by setting a fixed window size using `windowSetGeometryHints`. This seems to work for me and the Taffybar window does not expand past the monitor limit even with very long window titles shown (unfortunately in these cases the title widget overlaps the end widgets, however). You may have already considered and rejected `windowSetGeometryHints` as a solution to the problem and in that case I can rework the patch to use the hack.