Major overhaul of ModelForm.java:

1. Class and its nested classes were not thread-safe.
2. Bugs caused by C&P errors.
3. The "extends" feature had problems:
  3.1 Only a few class members were inherited from the parent form.
  3.2 Child forms modified the state of the parent form.
4. Bad null/empty checking.
5. Inefficient code.

git-svn-id: https://svn.apache.org/repos/asf/ofbiz/trunk@1636339 13f79535-47bb-0310-9956-ffa450edef68