Improve system registration syntax

[AndreaCatania]

Currently the system are registered with the following syntax:

ECS::register_system(my_system_func, "MySystemFunc")
        	.set_description("Description")
        	.set_phase(PHASE_CONFIG))

The fact to force overriding the function name by default isn't a good syntax and would be better make it optional via function. In addition, the set_ prefix isn't the best, and with_ sounds much better.

ECS::register_system(my_system_func)
        	.with_name("MySystemFunc")
        	.with_description("Description")
        	.set_phase(PHASE_CONFIG))

[AndreaCatania]

Moving the proposal to discussions to keep the issue just for things to fix.