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.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <AK/Badge.h>
+#include <AK/OwnPtr.h>
+#include <AK/Vector.h>
+#include <AK/WeakPtr.h>
+
+class GWidget;
+
+class GLayout {
+public:
+    GLayout();
+    virtual ~GLayout();
+
+    void add_widget(GWidget&);
+    void add_layout(OwnPtr<GLayout>&&);
+
+    virtual void run(GWidget&) = 0;
+
+    void notify_adopted(Badge<GWidget>, GWidget&);
+    void notify_disowned(Badge<GWidget>, GWidget&);
+
+protected:
+    struct Entry {
+        WeakPtr<GWidget> widget;
+        OwnPtr<GLayout> layout;
+    };
+    WeakPtr<GWidget> m_owner;
+    Vector<Entry> m_entries;
+};
+