Implemented TimedLife trait.

Closes #5724.

Replaces RemovesImmediately.
If the ticks are left on the default of 0, it acts exactly like RemovesImmediately.
I tested using ```self.Kill()``` in ```AddedToWorld```, appears not to do anything. ```.Destroy()``` works fine here though.
I'll check again. Might have done something stupid first time around.