[IZPACK-1146] GUI Installer - UserInputPanel radiobutton/checkbox variables set even though the panel hasn't been visited

This has introduced since 5.0.0-rc2:

For UserInputPanels in GUI/Swing installers the variables for radiobuttons and checkboxes are already set to the default values although the panel hasn't been reached. This is not the behavior like originally designed in IzPack. Variables should be unset as long as the panel that sets them hasn't been left or as long as an explicit variable or dynamicvariable definition doesn't set them explicitly.

This breaks existing installers in a nasty way, especially if there are conditions bound to such variables that rely on the original behavior.