LibGUI: Add GLazyWidget, a convenience widget for lazily-built UI's

Here's how you can use this to speed up startup time:

    auto widget = GLazyWidget::construct();
    widget->on_first_show = [](auto& self) {
        self.set_layout(...);
        ...
    };

Basically, it allows you to delay building the widget subtree until
it's shown for the first time.

Libraries/LibGUI/GLazyWidget.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class GLazyWidget : public GWidget {
+    C_OBJECT(GLazyWidget)
+public:
+    virtual ~GLazyWidget() override;
+
+    Function<void(GLazyWidget&)> on_first_show;
+
+protected:
+    explicit GLazyWidget(GWidget* parent = nullptr);
+
+private:
+    virtual void show_event(GShowEvent&) override;
+
+    bool m_has_been_shown { false };
+};