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/GSpinBox.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class GButton;
+class GTextEditor;
+
+class GSpinBox : public GWidget {
+public:
+    GSpinBox(GWidget* parent = nullptr);
+    virtual ~GSpinBox() override;
+
+    int value() const { return m_value; }
+    void set_value(int);
+
+    int min() const { return m_min; }
+    int max() const { return m_max; }
+    void set_min(int min) { set_range(min, max()); }
+    void set_max(int max) { set_range(min(), max); }
+    void set_range(int min, int max);
+
+    Function<void(int value)> on_change;
+
+    virtual const char* class_name() const override { return "GSpinBox"; }
+
+protected:
+    virtual void resize_event(GResizeEvent&) override;
+
+private:
+    GTextEditor* m_editor { nullptr };
+    GButton* m_increment_button { nullptr };
+    GButton* m_decrement_button { nullptr };
+
+    int m_min { 0 };
+    int m_max { 100 };
+    int m_value { 0 };
+};