Add convenience methods

Author: alexeysemenyukoracle

src/jdk.jpackage/linux/classes/jdk/jpackage/internal/model/LinuxPackage.java

@@ -34,6 +34,8 @@
  */
 public interface LinuxPackage extends Package, LinuxPackageMixin {
 
+    LinuxApplication app();
+
     @Override
     AppImageLayout packageLayout();
 

src/jdk.jpackage/windows/classes/jdk/jpackage/internal/model/WinMsiPackage.java

@@ -28,6 +28,8 @@
 
 public interface WinMsiPackage extends Package, WinMsiPackageMixin {
 
+    WinApplication app();
+
     public static WinMsiPackage create(Package pkg, WinMsiPackageMixin mixin) {
         return CompositeProxy.create(WinMsiPackage.class, pkg, mixin);
     }