Support prompt_toolkit's in_thread

[MasterofJOKers]

We expose the functionality to run CliMenu in a background thread that the underlying prompt_toolkit library supports through a run_in_thread argument. This functionality can be necessary if we get called from a running event loop as the documentation of in_thread describes:

:param in_thread: When true, run the application in a background
	thread, and block the current thread until the application
	terminates. This is useful if we need to be sure the application
	won't use the current event loop (asyncio does not support nested
	event loops). A new event loop will be created in this background
	thread, and that loop will also be closed when the background
	thread terminates. When this is used, it's especially important to
	make sure that all asyncio background tasks are managed through
	`get_appp().create_background_task()`, so that unfinished tasks are
	properly cancelled before the event loop is closed. This is used
	for instance in ptpython.