LibGUI: Start adding an automatic widget layout system.

My needs are really quite simple, so I'm just going to add what I need
as I go along. The first thing I needed was a simple box layout with
widgets being able to say whether they prefer fixed or fill for both
their vertical and horizontal sizes.

I also made a simple GStatusBar so FileManager can show how many bytes
worth of files are in the current directory.

LibGUI/GLayout.cpp

@@ -0,0 +1,41 @@
+#include <LibGUI/GLayout.h>
+#include <LibGUI/GWidget.h>
+
+GLayout::GLayout()
+{
+}
+
+GLayout::~GLayout()
+{
+}
+
+void GLayout::notify_adopted(Badge<GWidget>, GWidget& widget)
+{
+    if (m_owner.ptr() == &widget)
+        return;
+    m_owner = widget.make_weak_ptr();
+}
+
+void GLayout::notify_disowned(Badge<GWidget>, GWidget& widget)
+{
+    ASSERT(m_owner.ptr() == &widget);
+    m_owner.clear();
+}
+
+void GLayout::add_layout(OwnPtr<GLayout>&& layout)
+{
+    Entry entry;
+    entry.layout = move(layout);
+    m_entries.append(move(entry));
+    if (m_owner)
+        m_owner->notify_layout_changed(Badge<GLayout>());
+}
+
+void GLayout::add_widget(GWidget& widget)
+{
+    Entry entry;
+    entry.widget = widget.make_weak_ptr();
+    m_entries.append(move(entry));
+    if (m_owner)
+        m_owner->notify_layout_changed(Badge<GLayout>());
+}