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/GStatusBar.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class GLabel;
+
+class GStatusBar : public GWidget {
+public:
+    explicit GStatusBar(GWidget* parent);
+    virtual ~GStatusBar() override;
+
+    String text() const;
+    void set_text(String&&);
+
+private:
+    virtual const char* class_name() const override { return "GStatusBar"; }
+    virtual void paint_event(GPaintEvent&) override;
+
+    GLabel* m_label { nullptr };
+};