Bug 163360 [Field Assist] - API - Provide ControlDecoration constructor without specifying decoration
Currently you can only create a ControlDecoration by specifying the control and the decoration in the constructor. This works nicely when the decoration is static (ie, always content assist). But there are use cases where the decoration changes depending on the state of the field, and at creation time, it's not clear which decoration will be set. Having a constructor that doesn't require a decoration would be nice.
bundles/org.eclipse.jface/src/org/eclipse/jface/fieldassist/ControlDecoration.java
*         always be shown.
	 */
	public boolean getShowOnlyOnFocus() {
		return showOnlyOnFocus;
	}
	/**
	 * Set the boolean that controls whether the decoration is shown only when
	 * the control has focus. The default value of this setting is
	 * <code>false</code>.
	 * 
	 * @param showOnlyOnFocus
	 *            <code>true</code> if the decoration should only be shown
	 *            when the control has focus, and <code>false</code> if it
	 *            should always be shown.
	 */
	public void setShowOnlyOnFocus(boolean showOnlyOnFocus) {
		this.showOnlyOnFocus = showOnlyOnFocus;
		update();
	}
	/**
	 * Get the boolean that controls whether the decoration's description text
	 * should be shown in a hover when the user hovers over the decoration. The
	 * default value of this setting is <code>true</code>.
	 * 
	 * @return <code>true</code> if a hover popup containing the decoration's
	 *         description text should be shown when the user hovers over the
	 *         decoration, and <code>false</code> if a hover should not be
	 *         shown.
	 */
	public boolean getShowHover() {