event: added method event.exists(event_name)

[da-h]

event.exists(event_name)

Attention: Using the event with the name exists is not possible anymore with this MR.

This MR adds the function event.exists(event_name) to each event object.

Description

Instead of calling event.optional.event_name one might want to check if an event exists and proceed from there with different behavior.

Check all before creating this PR:

• Documentation adapted
• unit tests adapted / created

Example Usage

It can be used inside events as follows:

def some_event(event):
   if event.exists("other_event"):
       print("other_event exists")

This can be also used for a more dynamic registration:

def register(mf):
   if mf.event.exists("some_event"):
      mf.register_event("some_event", lambda x: print("some_event called"))