Add a new natlink function for controlling the message window

[drmfinlay]

This was an idea that developed from my thinking on issue #198.

In a nutshell, an additional, setTrayIcon-like function could be added to enable/disable the Natlink message window and to handle user events (as preferred) in Python. This would allow us to hide the window for users in a multi-user system who don't have Natlink enabled and to use it in remote python.exe processes (at least for testing).

The function name I propose to use is setMessageWindow(). This function would take a single parameter: a callback function to handle user events. Adding a new File>Exit menu item to the window seems appropriate as part of this change. Ditto for detaching the window from the NatSpeak connection.

The following is the documentation I came up with:

setMessageWindow( callback )
This function enables and disables Natlink's message window. It may be called before or after natConnect().

This function has one required parameter whose value should be a Python function or None. If a function is passed, it should take one parameter which is the type of message window event:
idd_reload or idd_exit (each defined in natlinkutils)

Note: The idd_reload event occurs when the File>Reload menu item is activated. Before this function's callback is invoked, Natlink will unload all loaded grammars, destroy all existing ResObjs and DictObjs, and clear all callbacks except the message window callback (this one).

Passing a callback function enables the message window, passing None disables it. If None is passed and the message window exists, a File>Exit event (idd_exit) will be sent to the callback and the window will be destroyed.

I have this planned out and will try to make pull requests for review soon. I think it would be useful...

[drmfinlay]

I think I'll leave completely detaching the window from the NatSpeak connection for now. It is possible, but not strictly necessary here.