ScreenSaver prefs: Invoke list item runs test

This copies the behavior of BeOS R5 ScreenSaver prefs.

Don't invoke the listview on AllAttached() anymore, this would make
the selected screensaver start when the pref window opens.

Horizontally center the Test button.

Change-Id: I65471112d37d9efd17945f05bce485ff2fece9dd
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1958
Reviewed-by: Adrien Destugues <[email protected]>

src/preferences/screensaver/ScreenSaverWindow.cpp

@@ -577,6 +577,8 @@ ModulesView::ModulesView(const char* name, ScreenSaverSettings& settings)
 
 	fScreenSaversListView->SetSelectionMessage(
 		new BMessage(kMsgSaverSelected));
+	fScreenSaversListView->SetInvocationMessage(
+		new BMessage(kMsgTestSaver));
 	BScrollView* saversListScrollView = new BScrollView("scroll_list",
 		fScreenSaversListView, 0, false, true);
 
@@ -592,10 +594,7 @@ ModulesView::ModulesView(const char* name, ScreenSaverSettings& settings)
 		.AddGroup(B_VERTICAL)
 			.Add(fPreviewView)
 			.Add(saversListScrollView)
-			.AddGroup(B_HORIZONTAL)
-				.Add(fTestButton)
-				.AddGlue()
-				.End()
+			.Add(fTestButton)
 			.End()
 		.Add(fSettingsBox)
 		.End();
@@ -634,7 +633,6 @@ void
 ModulesView::AllAttached()
 {
 	PopulateScreenSaverList();
-	fScreenSaversListView->Invoke();
 }