Example for DrawingDecorator

[EHAT32]

Added example so that methods from trueagi-io/Vereya#78 could be tested. Also enabled drawingdecorators in ServerHandlers

@CICS-Oleg

[EHAT32]

Now added a way to create Draw objects which can be put into mission builder and created a simple example for drawing objects. Also, made a minor update for method init_mission in common.py. Now there's an option to create a default or flat world as parameter. It was done in order to be able import one method for every unit test instead of copying and pasting the same method in code when the only change is a world type.