Bug 294462 [FieldAssist] ControlDecoration#showHoverText doesn't check if the widget is visible
If the HOVER-Text is set on decoration of a widget which is invisible the Hover-Shell is still poping up because it doesn't check if the control is invisible make the Hover appearing somewhere on the screen.
bundles/org.eclipse.jface/src/org/eclipse/jface/fieldassist/ControlDecoration.java
* Adds the listener to the collection of listeners who will be notified
	 * when the decoration is selected, by sending it one of the messages
	 * defined in the <code>SelectionListener</code> interface.
	 * <p>
	 * <code>widgetSelected</code> is called when the decoration is selected
	 * (by mouse click). <code>widgetDefaultSelected</code> is called when the
	 * decoration is double-clicked.
	 * </p>
	 * <p>
	 * The <code>widget</code> field in the SelectionEvent will contain the
	 * Composite on which the decoration is rendered that received the click.
	 * The <code>x</code> and <code>y</code> fields will be in coordinates
	 * relative to that widget. The <code>data</code> field will contain the
	 * decoration that received the event.
	 * </p>
	 * 
	 * @param listener
	 *            the listener which should be notified
	 * 
	 * @see org.eclipse.swt.events.SelectionListener
	 * @see org.eclipse.swt.events.SelectionEvent
	 * @see #removeSelectionListener
	 */
	public void addSelectionListener(SelectionListener listener) {
		selectionListeners.add(listener);
	}
	/**
	 * Removes the listener from the collection of listeners who will be
	 * notified when the decoration is selected.
	 *