Takes into account NullValue
Uses comboboxes from widget factory
Revert "Uses AbstractIWindow as parent class for AbstractNavTable"

This reverts commit 9d985d6465433cb8e79b3276d31cddf99ba2c01d.

Conflicts:
	src/es/udc/cartolab/gvsig/navtable/AbstractNavTable.java
Changes how the logger is declared