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.cpp

@@ -0,0 +1,63 @@
+#include <LibGUI/GIcon.h>
+
+GIcon::GIcon()
+    : m_impl(GIconImpl::create())
+{
+}
+
+GIcon::GIcon(const GIconImpl& impl)
+    : m_impl(const_cast<GIconImpl&>(impl))
+{
+}
+
+GIcon::GIcon(const GIcon& other)
+    : m_impl(other.m_impl.copy_ref())
+{
+}
+
+GIcon::GIcon(RetainPtr<GraphicsBitmap>&& bitmap)
+    : GIcon()
+{
+    if (bitmap) {
+        ASSERT(bitmap->width() == bitmap->height());
+        int size = bitmap->width();
+        set_bitmap_for_size(size, move(bitmap));
+    }
+}
+
+GIcon::GIcon(RetainPtr<GraphicsBitmap>&& bitmap1, RetainPtr<GraphicsBitmap>&& bitmap2)
+    : GIcon(move(bitmap1))
+{
+    if (bitmap2) {
+        ASSERT(bitmap2->width() == bitmap2->height());
+        int size = bitmap2->width();
+        set_bitmap_for_size(size, move(bitmap2));
+    }
+}
+
+const GraphicsBitmap* GIconImpl::bitmap_for_size(int size) const
+{
+    auto it = m_bitmaps.find(size);
+    if (it != m_bitmaps.end())
+        return it->value.ptr();
+
+    int best_diff_so_far = INT32_MAX;
+    const GraphicsBitmap* best_fit = nullptr;
+    for (auto& it : m_bitmaps) {
+        int abs_diff = abs(it.key - size);
+        if (abs_diff < best_diff_so_far) {
+            best_diff_so_far = abs_diff;
+            best_fit = it.value.ptr();
+        }
+    }
+    return best_fit;
+}
+
+void GIconImpl::set_bitmap_for_size(int size, RetainPtr<GraphicsBitmap>&& bitmap)
+{
+    if (!bitmap) {
+        m_bitmaps.remove(size);
+        return;
+    }
+    m_bitmaps.set(size, move(bitmap));
+}