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/GBoxLayout.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <LibGUI/GLayout.h>
+#include <LibGUI/GWidget.h>
+
+class GBoxLayout final : public GLayout {
+public:
+    explicit GBoxLayout(Orientation);
+    virtual ~GBoxLayout() override;
+
+    Orientation orientation() const { return m_orientation; }
+
+    virtual void run(GWidget&) override;
+
+private:
+    Orientation m_orientation;
+};
+