Make DPIZoomChangeHandlers concurrent R/W able #62

This commit changes the type of
DPIZoomChangeRegistry::dpiZoomChangeHandlers from TreeMap to
ConcurrentSkipListMap as at times there can be classes which can
register their handlers amidst a DPI change event leading to concurrent
read and write which can lead to ConcurrentModificationException

contributes to #62 and #127

Author: amartya4256

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/internal/DPIZoomChangeRegistry.java

@@ -15,13 +15,14 @@
 
 import java.util.*;
 import java.util.Map.*;
+import java.util.concurrent.*;
 
 import org.eclipse.swt.*;
 import org.eclipse.swt.widgets.*;
 
 public class DPIZoomChangeRegistry {
 
-	private static Map<Class<? extends Widget>, DPIZoomChangeHandler> dpiZoomChangeHandlers =  new TreeMap<>(
+	private static Map<Class<? extends Widget>, DPIZoomChangeHandler> dpiZoomChangeHandlers =  new ConcurrentSkipListMap<>(
 			(o1, o2) -> {
 	            if(o1.isAssignableFrom(o2)) {
 	            	return -1;