Manage per-screen status bars.

Provides a few helper functions to manage per-screen status bars while dynamically responding to screen changes. A startup action, event hook, and a way to separate PP styles based on the screen's focus are provided:

• The dynStatusBarStartup hook which initializes the status bars. The first argument is an `ScreenId -> IO Handle` which spawns a status bar on the given screen and returns the pipe which the string should be written to. The second argument is a `IO ()` to shut down all status bars. This should be placed in your startupHook.
• The dynStatusBarEventHook hook which respawns status bars when the number of screens changes. The arguments are the same as for the dynStatusBarStartup function. This should be placed in your handleEventHook.
• Each of the above functions have an alternate form (dynStatusBarStartup` and dynStatusBarEventHook`) which use a cleanup function which takes an additional ScreenId argument which allows for more fine-grained control for shutting down a specific screen's status bar.
• The multiPP function which allows for different output based on whether the screen for the status bar has focus (the first argument) or not (the second argument). This is for use in your logHook.
• The multiPPFormat function is the same as the multiPP function, but it also takes in a function that can customize the output to status bars.

The hooks take a DynamicStatusBar function which is given the id of the screen to start up and returns the Handle to the pipe to write to. The DynamicStatusBarCleanup argument should tear down previous instances. It is called when the number of screens changes and on startup.