Systems now place all of their component factories into a `Components`
member. This improves code readability and makes it clear when we are
dealing specifically with ecs components.
The concrete ComponentFactory instances now have `Add` and `Get`
methods (as opposed to `AddAlpha` or `GetAlpha`). This enforces naming
of component factories as to avoid collisions when embedded in a struct
with other components.
Also, the ComponentFactory interface is embedded directly into the
concrete component factory without a name.
* added `d2common/d2input`, copied input vector logic from hellspawner
* added an `InteractiveComponent` which contains input vector, enable
flag, and callback function
* Added an InputSystem which handles input logic and iterates over
entities with interactive components
* added a mouse cursor scene for rendering the mouse cursor
* made the trademark sprite disappear when left mouse is clicked
* various other small bugfixes in scene systems
