Dialog should float

- New WindowView which can be partially atop of background.
- DialogView now extends WindowView
- Change dialog in Catalog sample and Dialog scenario
- Relates #825
- Relates #855

spring-shell-samples/spring-shell-sample-catalog/src/main/java/org/springframework/shell/samples/catalog/Catalog.java

@@ -295,7 +295,6 @@ public class Catalog {
 		dialog.setEventLoop(eventLoop);
 		dialog.setViewService(ui);
 
-		dialog.setTransparent(false);
 		return dialog;
 	}
 

spring-shell-samples/spring-shell-sample-catalog/src/main/java/org/springframework/shell/samples/catalog/scenario/other/DialogScenario.java

@@ -51,7 +51,6 @@ public class DialogScenario extends AbstractScenario {
 		});
 		DialogView dialog = new DialogView(content, button);
 		configure(dialog);
-		dialog.setTransparent(false);
 		dialog.setViewService(getViewService());
 		return dialog;
 	}