Add Conveyor Belt object

[Raoul1808]

This PR adds conveyor belts, a new object that shifts entities in a certain direction at a certain rate.

They have variable length and speed and can shift entities left or right. The conveyor can also be controlled through a script to start or stop the conveyor, change its speed or direction.

Some things may need more polish, like for example extending the texture/animation so it can be seamless over longer belts.

Any feedback is welcome, especially on the code. I'm very new to adding new objects into the game.

[mrkubax10]

Could you fix compilation issues?

[Raoul1808]

I just changed the way the conveyor belt shift objects, but there's still the issue of some enemies turning around at the end of the conveyor belt (it only affects enemies that stay on their platform), I don't really know how to tackle this

[Vankata453]

@weluvgoatz Is the PR good after the new changes?