Add support to the slider to format the value

Currently the slider always show the raw number, but often this is not
very suitable  for the user e.g. if the Number represents a unit of
measure or should be formated with a number format.

This adds a new method NebulaSlider#setLabelFormatProvider that allows
to customize the formating of the raw number.

widgets/nebulaslider/org.eclipse.nebula.widgets.nebulaslider/src/org/eclipse/nebula/widgets/opal/nebulaslider/NebulaSlider.java

@@ -12,6 +12,8 @@
  *******************************************************************************/
 package org.eclipse.nebula.widgets.opal.nebulaslider;
 
+import java.util.function.IntFunction;
+
 import org.eclipse.nebula.widgets.opal.commons.SelectionListenerUtil;
 import org.eclipse.swt.SWT;
 import org.eclipse.swt.SWTException;
@@ -47,6 +49,8 @@ public class NebulaSlider extends Canvas {
 
 	private boolean moving = false;
 
+	private IntFunction<String> format;
+
 	/**
 	 * Constructs a new instance of this class given its parent and a style value
 	 * describing its behavior and appearance.
@@ -181,7 +185,7 @@ private void drawSelector(final GC gc) {
 		// And the value
 		gc.setForeground(renderer.getSelectorTextColor());
 		gc.setFont(renderer.getTextFont());
-		final String valueAsString = String.valueOf(value);
+		final String valueAsString = stringValueOf(value);
 		final Point textSize = gc.textExtent(valueAsString);
 
 		final int xText = hMargin + xPosition + selectorWidth / 2;
@@ -190,6 +194,23 @@ private void drawSelector(final GC gc) {
 		gc.drawText(valueAsString, xText - textSize.x / 2, yText - textSize.y / 2, true);
 	}
 
+	private String stringValueOf(int value) {
+		if(format != null) {
+			return format.apply(value);
+		}
+		return String.valueOf(value);
+	}
+
+	/**
+	 * Set the format that should be used to format the given number to a string.
+	 * 
+	 * @param format
+	 *                   the format or <code>null</code> to use the default format.
+	 */
+	public void setLabelFormatProvider(IntFunction<String> format) {
+		this.format = format;
+	}
+
 	private void addMouseListeners() {
 
 		addListener(SWT.MouseDown, e -> {