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.cpp

@@ -0,0 +1,89 @@
+#include <LibGUI/GBoxLayout.h>
+#include <LibGUI/GWidget.h>
+
+GBoxLayout::GBoxLayout(Orientation orientation)
+    : m_orientation(orientation)
+{
+}
+
+GBoxLayout::~GBoxLayout()
+{
+}
+
+#if 0
+Size GLayout::compute_preferred_size() const
+{
+
+}
+
+
+static Size compute_preferred_size(GLayout::Entry& entry)
+{
+    if (entry.layout)
+        return entry.layout->compute_preferred_size();
+    else {
+        return entry.widget->preferred_size();
+    }
+}
+#endif
+
+void GBoxLayout::run(GWidget& widget)
+{
+    if (m_entries.is_empty())
+        return;
+
+    Size available_size = widget.size();
+    int number_of_entries_with_fixed_size = 0;
+
+    for (auto& entry : m_entries) {
+        if (entry.widget && entry.widget->size_policy(orientation()) == SizePolicy::Fixed) {
+            available_size -= entry.widget->preferred_size();
+            ++number_of_entries_with_fixed_size;
+        }
+    }
+
+    int number_of_entries_with_automatic_size = m_entries.size() - number_of_entries_with_fixed_size;
+
+    dbgprintf("GBoxLayout: available_size=%d, fixed=%d, fill=%d\n", available_size.height(), number_of_entries_with_fixed_size, number_of_entries_with_automatic_size);
+
+    Size automatic_size;
+
+    if (m_orientation == Orientation::Horizontal) {
+        automatic_size.set_width(available_size.width() / number_of_entries_with_automatic_size);
+        automatic_size.set_height(widget.height());
+    } else {
+        automatic_size.set_width(widget.width());
+        automatic_size.set_height(available_size.height() / number_of_entries_with_automatic_size);
+    }
+
+    dbgprintf("GBoxLayout: automatic_size=%s\n", automatic_size.to_string().characters());
+
+    int current_x = 0;
+    int current_y = 0;
+ for (auto& entry : m_entries) {
+        Rect rect(current_x, current_y, 0, 0);
+        if (entry.layout) {
+            // FIXME: Implement recursive layout.
+            ASSERT_NOT_REACHED();
+        }
+        ASSERT(entry.widget);
+        if (entry.widget->size_policy(orientation()) == SizePolicy::Fixed) {
+            rect.set_size(automatic_size);
+            if (orientation() == Orientation::Vertical) {
+                rect.set_height(entry.widget->preferred_size().height());
+            } else {
+                rect.set_width(entry.widget->preferred_size().height());
+            }
+        } else {
+            rect.set_size(automatic_size);
+        }
+
+        dbgprintf("GBoxLayout: apply, %s{%p} <- %s\n", entry.widget->class_name(), entry.widget.ptr(), rect.to_string().characters());
+        entry.widget->set_relative_rect(rect);
+
+        if (orientation() == Orientation::Horizontal)
+            current_x += rect.width();
+        else
+            current_y += rect.height();
+    }
+}