[SDL Widget] Implement 'setState()' to provide a place to wipe the texture for re-rendering.
[SDL Widgets] Utilize setting texture = 0 to force a re-render of the widget.
[makefile (root)] Add entity-manager.cpp. Fix an issue with the linker looking in obj/obj/
[Entity] Reorder data members to match initialization. Update constructor format for readability. Add new constructor.

Also, added newline at end of file.
[Entity] Split initialization across multiple lines for readability.