PDFBOX-1915: refactoring: base class and intermediate class (for types 4 5 6 7) to get rid of double code

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1616895 13f79535-47bb-0310-9956-ffa450edef68
PDFBOX-1915: forgot to delete declaration of variable that is now in base class

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1616907 13f79535-47bb-0310-9956-ffa450edef68