[Webtest] Adapt classes for extension

[Marc Guillemot]

Hi,

the changes made to facilitate implementation of custom steps or custom
changes to existing steps seem great. Yet the step classes could be improved
to facilitate extension.

Ex: in com.canoo.webtest.steps.StoreXPath

2 member properties:
    private String fXPath = null;
    private String fProperty = null;

but
- only setters and no getters for these
- member methods set/get directly the values without to use setter (and non
existing getter)

Therefore a child class has to overwrite far more as it should if the class
com.canoo.webtest.steps.StoreXPath would have been designed for inheritance.

Marc.