Start, stop, query the default cli applicationSource:
start_app creates an app, and places it on the top of the app stack.
Theme to use.
How to print the output.
Whether to stop the app, when the calling frame is destroyed.
The environment to use, instead of the calling frame, to trigger the stop of the app.
App to stop. If
NULL, the current default app is stopped. Otherwise we find the supplied app in the app stack, and remote it, together with all the apps above it.
start_app returns the new app,
default_app returns the default app.
stop_app does not return anything.