Bug 129214 [Field Assist] - Decorated Field attachments not centering images horizontally
DecoratedField is supposed to center decorations horizontally on each side. The current attachments are not doing so. Also consider whether grid would have been a better choice for internal implementation.
bundles/org.eclipse.jface/src/org/eclipse/jface/fieldassist/DecoratedField.java
* and location of the hover to hover near the specified control,
		 * pointing the arrow toward the target control.
		 */
		void setText(String t, Control hoverNear, Control targetControl) {
			if (t == null)
				t = EMPTY;
			if (!t.equals(text)) {
				Point oldSize = getExtent();
				text = t;
				hoverShell.redraw();
				Point newSize = getExtent();
				if (!oldSize.equals(newSize)) {
					
					arrowOnLeft = hoverNear.getLocation().x <= targetControl
							.getLocation().x;
					setNewShape();
				}
			}
			if (hoverNear != null) {
				Point extent = getExtent();
				int y = -extent.y - hah + 1;
				int x = arrowOnLeft ? -hao + haw / 2 : -extent.x + hao + haw
						/ 2;
				hoverShell.setLocation(hoverNear.toDisplay(x, y));
			}
		}
		/*
		 * Return whether or not the hover (shell) is visible.
		 */
		boolean isVisible() {