Attribute Filters:
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Like filters that manipulate an element's value, attribute filter are
capable to alter element's attributes. Such a filter can by applied to 
an element and to form. Well, a form is nothing more than a bunch of 
elements in this case.

In case more than one attribute filter was applied to an element, all 
of them will be executed - in the same order as they were added. In other
words, each filter might recieve a value which has already been altered 
by one or more filters before.

The applied filters will be executed based on events. Hence the attribute 
filter base class implements methods named "on[SOMEEVENT]" which must be 
overwritten by the concreate attribute filter to actually do something. 

So far, attribute filters support two events: "apply" and "set", hence they
implement the member functions: "onApply" and "onSet".

Set Event
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Each time an element's attribute is set the "onSet" function will be called.


Apply Event
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The apply event happens as soon as an attribute filter was added to an 
element. In other words, the "onApply" function is executed.

One might think, that this event is quite unnecessary. Because the attributes
will be initually set, the "set" event happens at time of element creation. Yes, 
it is true, that the "set" event happens. Unfortunately there can't be any 
attribute filters at this stage of an element's life cycle. What happens is that
elements are created first and filters added later.