Entity parents -> entities moving others around

Moving platforms and such just became reality with this commit! It adds
& changes a lot of things to provide the following features:
* Entities have parents (interface IEntityParent). If no other is
defined, the level itself is used.
* Entity coordinates are now in relative coordinate space. The relative
coordinates are kept private though, to harmonize with this:
* The parents provide a recursive framework for obtaining the absolute
location of an entity. The entity itself has a method getLocation()
which is a wrapper around it.
* Each entity can also be the parent of another entity. Entities have
preconfigured options for automatically parenting one another when
certain circumstances are met. They can also disallow parenting of
themselves.

Additionally, these things were implemented into the API:
* Conversion from jangoDare.Vector to java.awt.Point (mainly for Shape
instantiation)
* Buffer for collision checks (necessary for entity parents, but maybe
useful in some other ways, too)

The entity parenting is not fully complete yet, but it works
sufficiently for the moment. Things to add are, for example:
* Collision between child and other collidables when parent moves
* Better movement of childs when on the parent (currently very slow and
choppy, possibly because of gravity force)