Summary
-------

Make several AWT API classes sealed or final.

Problem
-------

There are classes in the AWT API which have a known set of direct sub-classes, or no sub-classes. These should have the sealed or final modifier applied to indicate this.

Solution
--------

Apply the sealed modifier from JEP 409 ( https://openjdk.java.net/jeps/409 )  to explicitly document and limit the direct API sub-classes of existing API classes which have no public or protected constructors - and so can be compatibly updated in this way. Similarly identified classes which have no sub-classes should be marked final. 

Sub-classes of newly sealed classes are made non-sealed as needed, or final if they are not already - if that is a compatible change.

Specification
-------------

java.awt.GridBagLayoutInfo

<pre>
-public class GridBagLayoutInfo implements java.io.Serializable
+public final class GridBagLayoutInfo implements java.io.Serializable
</pre>

--------------

java.awt.PointerInfo

<pre>
-public class PointerInfo 
+public final class PointerInfo 
</pre>

--------------

java.awt. ScrollPaneAdjustable

<pre>
-public class ScrollPaneAdjustable implements Adjustable, Serializable 
+public final class ScrollPaneAdjustable implements Adjustable, Serializable 
</pre>

-------
java.awt.dnd.DropTargetContext

<pre>

-public class DropTargetContext implements Serializable 
+public final class DropTargetContext implements Serializable
</pre>

-------------------

java.awt.TextComponent

<pre>
-public class TextComponent extends Component implements Accessible {
+public sealed class TextComponent extends Component implements Accessible
+     permits TextArea,
+             TextField {
</pre>

------

java.awt.TextField

<pre>
-public class TextField extends TextComponent 
+public non-sealed class TextField extends TextComponent 
</pre>

-------
java.awt.TextArea

<pre>
-public class TextArea extends TextComponent 
+public non-sealed class TextArea extends TextComponent 
</pre>

--------

java.awt.InputEvent

<pre>
-public abstract class InputEvent extends ComponentEvent 
+public abstract sealed class InputEvent extends ComponentEvent
+    permits KeyEvent,
+            MouseEvent {
</pre>

---------

java.awt.event.KeyEvent

<pre>
-public class KeyEvent extends InputEvent 
+public non-sealed class KeyEvent extends InputEvent 
 </pre>

------

java.awt.event.MouseEvent

<pre>
-public class MouseEvent extends InputEvent 
+public non-sealed class MouseEvent extends InputEvent 
</pre>


------

java.awt.desktop.AppEvent

<pre>
-public class AppEvent extends EventObject {
+public sealed class AppEvent extends EventObject
+    permits AboutEvent,
+            AppForegroundEvent,
+            AppHiddenEvent,
+            AppReopenedEvent,
+            FilesEvent,
+            OpenURIEvent,
+            PreferencesEvent,
+            QuitEvent,
+            ScreenSleepEvent,
+            SystemSleepEvent,
+            UserSessionEvent {
</pre>


--------


java.awt.desktop.FilesEvent

<pre>
-public class FilesEvent extends AppEvent {
+public sealed class FilesEvent extends AppEvent
+    permits OpenFilesEvent,
+            PrintFilesEvent {
</pre>

---------