Libraries: Create top level directory for libraries.

Things were getting a little crowded in the project root, so this patch
moves the Lib*/ directories into Libraries/.

Libraries/LibGUI/GLayout.h

@@ -0,0 +1,53 @@
+#pragma once
+
+#include <AK/Badge.h>
+#include <AK/OwnPtr.h>
+#include <AK/Vector.h>
+#include <AK/WeakPtr.h>
+#include <LibGUI/GMargins.h>
+
+class GWidget;
+
+class GLayout {
+public:
+    GLayout();
+    virtual ~GLayout();
+
+    void add_widget(GWidget&);
+    void add_layout(OwnPtr<GLayout>&&);
+    void add_spacer();
+
+    void remove_widget(GWidget&);
+
+    virtual void run(GWidget&) = 0;
+
+    void notify_adopted(Badge<GWidget>, GWidget&);
+    void notify_disowned(Badge<GWidget>, GWidget&);
+
+    GMargins margins() const { return m_margins; }
+    void set_margins(const GMargins&);
+
+    int spacing() const { return m_spacing; }
+    void set_spacing(int);
+
+protected:
+    struct Entry {
+        enum class Type {
+            Invalid = 0,
+            Widget,
+            Layout,
+            Spacer,
+        };
+
+        Type type { Type::Invalid };
+        WeakPtr<GWidget> widget;
+        OwnPtr<GLayout> layout;
+    };
+    void add_entry(Entry&&);
+
+    WeakPtr<GWidget> m_owner;
+    Vector<Entry> m_entries;
+
+    GMargins m_margins;
+    int m_spacing { 4 };
+};