BootManager: add a permanent "Quit" button.

As suggested on the mailing list by stippi and korli, it is important to
let the users know they can quit at any time, and it's sometimes the
only thing they can do.

src/apps/bootmanager/WizardView.cpp

@@ -125,6 +125,8 @@ WizardView::_BuildUI()
 		new BMessage(kMessagePrevious));
 	fNext = new BButton("next", B_TRANSLATE_COMMENT("Next", "Button"),
 		new BMessage(kMessageNext));
+	BButton* quit = new BButton("quit", B_TRANSLATE_COMMENT("Quit", "Button"),
+		new BMessage(B_QUIT_REQUESTED));
 
 	BLayoutBuilder::Group<>(this)
 		.Add(fPageContainer)
@@ -132,6 +134,7 @@ WizardView::_BuildUI()
 		.AddGroup(B_HORIZONTAL)
 			.SetInsets(B_USE_DEFAULT_SPACING, B_USE_DEFAULT_SPACING,
 				B_USE_DEFAULT_SPACING, B_USE_DEFAULT_SPACING)
+			.Add(quit)
 			.AddGlue()
 			.Add(fPrevious)
 			.Add(fNext)