Add "receiveMessage()" into UPBGE

Since we have “sendMessage()” in UPBGE’s Python commands which allows for objects to send a message without requiring a Message actuator, it seems natural to me that there should be a “receiveMessage()” command as well, eliminating the requirement for a Message sensor.

The example code would look like this:

own.receiveMessage('sample_messsge')

This works like the sendMessage() function but for receiving messages without the Message sensor. You can use the bodies and propName commands with this too.

What do you all think?