Merge FileImageDescriptor into URLImageDescriptor and clean it up

In it's core the FileImageDescriptor queries the given context class for
a resource at the given path and then operates on the returned URL or
directly fetches the resource's stream. By optaining the resource's URL
immediately and using an URLImageDescriptor with it a lot of similar
code and logic from the FileImageDescriptor can be saved.

Additionally apply a few minor code clean-ups and remove a unused
internal method.

Author: HannesWell

bundles/org.eclipse.jface/src/org/eclipse/jface/resource/FileImageDescriptor.java

@@ -76,6 +76,9 @@ protected ImageDescriptor() {
 	ImageDescriptor(boolean shouldBeCached) {
 		super(shouldBeCached);
 	}
+
+	private static final ImageDescriptor NULL_IMAGE = createFromImageDataProvider(z -> null);
+
 	/**
 	 * Creates and returns a new image descriptor from a file.
 	 *
@@ -84,7 +87,11 @@ protected ImageDescriptor() {
 	 * @return a new image descriptor
 	 */
 	public static ImageDescriptor createFromFile(Class<?> location, String filename) {
-		return new FileImageDescriptor(location, filename);
+		URL url = location != null ? location.getResource(filename) : null;
+		if (url == null) {
+			return NULL_IMAGE; // Defer failure to the time when the image is created
+		}
+		return new URLImageDescriptor(url);
 	}
 
 	/**