Let a sensor be destroyed or unwelded

This is necessary so that sensors may be replaced by different ones
without having memory leaks.
Implement setRange in MusicPlayer

Changes:
- setRange can be used to set the range of the music player to a
different value than the default
- Extract anonymous class to a nested class for easy use inside
MusicPlayer
Automatically stop quacking when sound is finished

Set up a timer to call stopQuacking() after the quackSound has finished
playing

This doesn't trigger an error from the world being locked because it is
done on the same thread
Allow subclasses of Entity to set sprite origin

Needed to center the bodies of Player, which doesn't use the super
constructor that previously dealt with centering a body in its sprite
Extract constant for player body size and center a smaller body
Merge pull request #12 from SweatyReptile/develop

Some bug fixes and feature implementations