Base PicoVisitor to allow for easy visiting of only one type of component.

I couldn't find anything similar to this in Pico's code base.

Modivation: Make a simple PicoVisitor that allows for focusing on only one (or more) types of components.

Using the included BasePicoVisitor, the following code that collects all Containers in a hierarchy becomes possible:

//Collect a list of all containers in the pico
final List allContainers = new ArrayList();
PicoVisitor testingVisitor = new BasePicoVisitor() {
public void visitContainer(PicoContainer pico)

};
testingVisitor.traverse(rootPico);

HTH!
-Mike (R)