Add the RubLight object

[HybridDog]

It glows when it is rubbed, e.g. when Tux or a badguy walks on it.
Skidding makes it glow bright.

I think this object could be used to fix the dark crystal mine issue #1317.

I have never added a new object to SuperTux, so please tell me if I did something wrong.

It has not much to do with that Rublight. I've given it this name because Triboluminescence literally means rublight and Triboluminescence is probably more complicated and realistic than what I have implemented.

[weluvgoatz]

Is the color of the light here configurable? If not, it would be nice to have.

[HybridDog]

Is the color of the light here configurable? If not, it would be nice to have.

I've made the color and fading speed configurable.
The default value of the fading_speed appears now in three places: the constructor, in the mapping.get invocation and in the get_settings method. I've looked at the spotlight code for how to do it and I don't know if the default value is required at all three places.

Edit: get_settings is used by the editor and mapping.get has an optional default value parameter.

[tobbi]

One small thing from Travis:

/rublight.cpp.o -c /Users/travis/build/SuperTux/supertux/src/object/rublight.cpp

/Users/travis/build/SuperTux/supertux/src/object/rublight.cpp:34:3: error: field 'fading_speed' will be initialized after field 'light' [-Werror,-Wreorder]

  fading_speed(5.0f),

  ^

1 error generated.

[HybridDog]

One small thing from Travis:

it should be fixed now