Adds a equals method to the KeyValue class

It returns true if a String is passed as parameter to the method and
is equal to the 'value' field of the KeyValue instance. Or if a KeyValue
is passed and 'value' and 'key' fields are equals in both objects
Removes onPositionChange from AbstractForm.

In this listener, AbstractForm was calling fillSpecificValues, but, this method was also called in super (super.onPositionChange -> refreshGUI -> fillValues
-> fillSpecificValues) This was causing troubles in corner cases, for example
when a combobox is modified in the fillSpecificValues of the form, because
the second time is called by means of the (removed) onPositionChange, the
fillValues flag was set to false, the combobox listener is launched and weird
behaviours happens