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/Makefile

@@ -55,6 +55,7 @@ OBJS = \
     GJsonArrayModel.o \
     GAboutDialog.o \
     GModelSelection.o \
+    GLazyWidget.o \
     GWindow.o
 
 LIBRARY = libgui.a