Core: Add a way to delay-construct mode widgets

... and use it for edit mode.

There's currently no real effect as the mode bar population in
ModeManagerPrivate::extensionsInitializedHelper() accesses all
IMode::widget() and triggers the creation, but in principle we
could pass the functor instead of the widget down to and store in
FancyTabWidget and only use it when a mode gets activated.

Change-Id: I4c4a276bc025abce1ff47c68b060c67b5c8e5170
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/debugger/debuggerplugin.cpp

@@ -500,13 +500,11 @@ public:
         setPriority(85);
         setId(MODE_DEBUG);
 
-        setWidget(new DebugModeWidget);
+        setWidgetCreator([] { return new DebugModeWidget; });
         setMainWindow(DebuggerMainWindow::instance());
 
         setMenu(&DebuggerMainWindow::addPerspectiveMenu);
     }
-
-    ~DebugMode() { delete widget(); }
 };
 
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