Provide a method in the event loop to 'process events' or 'do one iteration'

Many libs that has an event loop provide a method to ask the event loop to process all pending events and then return, for example, qt's QCoreApplication::processEvents(), matplotlib's canvas.flush_events(), ipython kernel's kernel.do_one_iteration(). Such a method is very convenient when integrating multiple event loops in a big program. Maybe I missed it, I couldn't find something equivalent in prompt-toolkit yet.

Start by locating prompt-toolkit's event loop implementation and its public entry points, then compare the requested behavior with QCoreApplication::processEvents(), canvas.flush_events(), and kernel.do_one_iteration(). Done means the event loop exposes a documented way to process pending events and return for integration with another event loop.