Make "content" the only property of GeometryElement.label

This moves the hiddenByParent property from GeometryElement.label to GeometryElement.label.content and removes the obsolete relativeCoords property. The labelcolor attribute is now using label.content.visProp.strokecolor to store its value.
Move GeometryElement.label.content to GeometryElement.label

Introducing a reference to the label in label.content to provide backwards compatibility is not possible because the JXG.Text element already has a content property.
Adjust the JC parser to use GeometryElement.label
Merge branch 'fix-gh-34'

This fixes #34