Continued with project:

Continued with project:
- Added new class BaseCategory and ProductCategory
- Implemented newly required method emptyStringToNull() in category database frontend
- Half-implemented newly required method emptyStringToNull() in product database frontend (unfinished)
- Category is now Comparable and asks for compareTo() method as well
- Added method addCategory() to database frontend
- doAdminAddCategory() should be "basicly finished" by now
- Better implement generateLinkForParent() to generate a HTML link for parent id. This links to product.jsp?category_id=x
Signed-off-by:Roland Häder <roland@mxchange.org>