First change enabling the the ability to provide customized panel to jbox2d test bed panel.

Our own test panel ui will be now from VertexTestPanel.java which is new file.
This change mostly is about refactoring TestbedSidePanel.java so that 
we can easily create customized ui control panel for our test.
For now, if you want to go back to the previous panel, simple change these lines in TestbedMain.java.

    //TestbedSidePanel side = new TestbedSidePanel(model, controller);
    TestbedSidePanel side = new VertexTestPanel(model, controller);

to 

    TestbedSidePanel side = new TestbedSidePanel(model, controller);
    //TestbedSidePanel side = new VertexTestPanel(model, controller);