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.cpp

@@ -0,0 +1,35 @@
+#include <LibGUI/GStatusBar.h>
+#include <LibGUI/GLabel.h>
+#include <LibGUI/GBoxLayout.h>
+#include <SharedGraphics/Painter.h>
+
+GStatusBar::GStatusBar(GWidget* parent)
+    : GWidget(parent)
+{
+    set_size_policy(SizePolicy::Fill, SizePolicy::Fixed);
+    set_preferred_size({ 0, 16 });
+    set_layout(make<GBoxLayout>(Orientation::Horizontal));
+    m_label = new GLabel(this);
+    m_label->set_fill_with_background_color(false);
+}
+
+GStatusBar::~GStatusBar()
+{
+}
+
+void GStatusBar::set_text(String&& text)
+{
+    m_label->set_text(move(text));
+}
+
+String GStatusBar::text() const
+{
+    return m_label->text();
+}
+
+void GStatusBar::paint_event(GPaintEvent&)
+{
+    Painter painter(*this);
+    painter.fill_rect({ 0, 1, width(), height() - 1 }, Color::LightGray);
+    painter.draw_line({ 0, 0 }, { width() - 1, 0 }, Color::DarkGray);
+}