Fairly substantial refactoring of preference system. Created abstract class net...

Fairly substantial refactoring of preference system.  Created abstract class net.sf.openrocket.startup.Preferences which defines abstract get/put primitive functions and some conversion utilities which are not dependent on swing/awt.  The class net.sf.openrocket.util.Prefs extends this to support execution in a swing application environment using Java system preferences (like it used to).  Prefs still contains some utility functions to convert to swing/awt types such as Dimension, Position, and java.awt.Color.

The net.sf.openrocket.util.Color class was added to facilitate making the core RocketComonent classes independent of swing/awt.  Conversion between net.sf.openrocket.util.Color and java.awt.Color is done by static methods in net.sf.openrocket.gui.util.ColorConversion.

git-svn-id: https://openrocket.svn.sourceforge.net/svnroot/openrocket/trunk@220 180e2498-e6e9-4542-8430-84ac67f01cd8