make ModalWindow CSS contibution more configurable

Right now if you want to modify the CSS of ModalWindow you still get added to page the original CSS as it is included in

/**

• Initialize
  */
  private void init() { setVersioned(false); cookieName = null; add(empty = new WebMarkupContainer(getContentId())); add(newCloseButtonBehavior()); add(new WindowClosedBehavior()); add(JavascriptPackageResource.getHeaderContribution(JAVASCRIPT)); add(CSSPackageResource.getHeaderContribution(CSS)); }

I propose to do this as

/**

• Initialize
  */
  private void init() { setVersioned(false); cookieName = null; add(empty = new WebMarkupContainer(getContentId())); add(newCloseButtonBehavior()); add(new WindowClosedBehavior()); add(JavascriptPackageResource.getHeaderContribution(JAVASCRIPT)); ResourceReference CSS = newCssResource(); if (CSS != null) add(CSSPackageResource.getHeaderContribution(CSS)); }

/**

• Allows to override CSS contribution. Returning null means the CSS will be contributed via
• other sources, e.g. a global CSS resource.
• @return The CSS resource reference or null if CSS is contributed via other means.
  */
  protected ResourceReference newCssResource() { return CSS; }

This will allow to easily override the CSS file.... or avoid including one (returning null)... in case the CSS is part of a "global" page CSS... Existing applications will not be affected