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.cpp

@@ -0,0 +1,20 @@
+#include <LibGUI/GLazyWidget.h>
+
+GLazyWidget::GLazyWidget(GWidget* parent)
+    : GWidget(parent)
+{
+}
+
+GLazyWidget::~GLazyWidget()
+{
+}
+
+void GLazyWidget::show_event(GShowEvent&)
+{
+    if (m_has_been_shown)
+        return;
+    m_has_been_shown = true;
+
+    ASSERT(on_first_show);
+    on_first_show(*this);
+}