Clean up the util package

[seifertm]

The package de.erichseifert.gral.util contains a collection of useful classes, although some of the should be moved to another package.

• Move de.erichseifert.gral.util.Insets2D, de.erichseifert.gral.util.Location, de.erichseifert.gral.util.Orientation to de.erichseifert.gral.graphics
• Move SerializationUtils and associated classes to the new package de.erichseifert.de.io.serialization

These changes are part of the API review (see issue #11)