Issue 1398 fix throwable bug with coins

[martinvanzijl]

Fixes #1398. Throwable objects like rocks will no longer collide with coins or "flame" objects, like rotating ice balls.

Demo before:

Demo after:

[martinvanzijl]

Should the idea of thrown objects collecting coins (#1398 (comment)) be raised as a seperate feature request?

[HybridDog]

There could be an easier fix.
As far as I can see, the cases in which the rock should not be moved are objects which have the COLGROUP_TOUCHABLE collision group, which is visualized in orange:

When I wrote the code I didn't think about touch collisions. I assume the problem could be fixed with moving_object->get_group() == COLGROUP_TOUCHABLE (and including collision/collision_group.hpp).

[martinvanzijl]

@HybridDog Thanks for the suggestion! This seems to work, and is a lot simpler, so I changed the code accordingly.