Add LiveText and Terminal behavior NVDAObjects to facilitate automatic reporting...

Add LiveText and Terminal behavior NVDAObjects to facilitate automatic reporting of new text for objects from which text can be retrieved and which notify of text changes, but do not specify the exact new text.
A background monitor thread is notified when the text changes and calculates the changed text using difflib.
This code is based on the monitor code in winConsoleHandler, but has been abstracted and significantly cleaned up. A future commit will probably start using this new code for winConsole.