Disposable resources must not be tracked via a set, since that will
remove images created multiple times.

java.lang.Error: SWT Resource was not properly disposed
	at org.eclipse.swt.graphics.Resource.initNonDisposeTracking(Resource.java:172)
	at org.eclipse.swt.graphics.Resource.<init>(Resource.java:120)
	at org.eclipse.swt.graphics.Image.<init>(Image.java:668)
	at org.eclipse.jface.resource.URLImageDescriptor.createImage(URLImageDescriptor.java:241)
	at org.eclipse.jface.resource.ImageDescriptor.createImage(ImageDescriptor.java:290)
	at org.eclipse.jface.resource.ImageDescriptor.createImage(ImageDescriptor.java:268)
	at org.eclipse.ui.internal.dialogs.ContentTypesPreferencePage.lambda$2(ContentTypesPreferencePage.java:363)