Bug 202316 [Forms] Add gradient sharing support for FormHeading
bundles/org.eclipse.ui.forms/src/org/eclipse/ui/internal/forms/widgets/FormImages.java
private static FormImages instance;
	public static FormImages getInstance() {
		if (instance == null)
			instance = new FormImages();
		return instance;
	}
	private HashMap images;
	private HashMap ids;
	
	private FormImages() {
	}
	
	private class ImageIdentifier {
		private Display fDisplay;
		private Color fColor1;
		private Color fColor2;
		private int fRealtheight;
		private int fTheight;
		private int fMarginHeight;
		
		ImageIdentifier (Display display, Color color1, Color color2,
				int realtheight, int theight, int marginHeight) {
			fDisplay = display;