Add way to group systems

[waynemwashuma]

Genesis.

During development of some plugins,i realised some systems share the same/similar functionality but operate on different components.

Description.

Systems with similar or the same goal should be grouped and run in together.

These systems this will also allow systems from different plugins to run together.

Solution(s).

Add system sets which can be registered into the App and systems can be added to them when registered.

More information.

This needs to be compatible with the feature requested by:

• Add ability to order systems explicitly. #24