Improves, refactors and creates new classes.

Improves Connector class: Now there can be one-to-multiple connections.

Refactors GUI class: Changed visibility of SWT shell from protected to
private as can be received via #getShell() method.

Creates new classes, one that holds a Pair of two different objects and
the other that helps with SWT GUIs.

Known Bugs: Groups are not positioned right by the GUITools class.