LibGUI+FileManager: Add a GIcon class to support multi-size icons.

A GIcon can contain any number of bitmaps internally, and will give you
the best fitting icon when you call bitmap_for_size().

LibGUI/GIcon.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <SharedGraphics/GraphicsBitmap.h>
+#include <AK/HashMap.h>
+
+class GIconImpl : public Retainable<GIconImpl> {
+public:
+    static Retained<GIconImpl> create() { return adopt(*new GIconImpl); }
+    ~GIconImpl() { }
+
+    const GraphicsBitmap* bitmap_for_size(int) const;
+    void set_bitmap_for_size(int, RetainPtr<GraphicsBitmap>&&);
+
+private:
+    GIconImpl() { }
+    HashMap<int, RetainPtr<GraphicsBitmap>> m_bitmaps;
+};
+
+class GIcon {
+public:
+    GIcon();
+    explicit GIcon(RetainPtr<GraphicsBitmap>&&);
+    explicit GIcon(RetainPtr<GraphicsBitmap>&&, RetainPtr<GraphicsBitmap>&&);
+    explicit GIcon(const GIconImpl&);
+    GIcon(const GIcon&);
+    ~GIcon() { }
+
+    GIcon& operator=(const GIcon& other)
+    {
+        m_impl = other.m_impl.copy_ref();
+        return *this;
+    }
+
+    const GraphicsBitmap* bitmap_for_size(int size) const { return m_impl->bitmap_for_size(size); }
+    void set_bitmap_for_size(int size, RetainPtr<GraphicsBitmap>&& bitmap) { m_impl->set_bitmap_for_size(size, move(bitmap)); }
+
+    const GIconImpl& impl() const { return *m_impl; }
+
+private:
+    Retained<GIconImpl> m_impl;
+};