LibGfx+LibIPC: Add Gfx::ShareableBitmap, a bitmap for easy IPC usage

With this patch, it's now possible to pass a Gfx::ShareableBitmap in an
IPC message. As long as the message itself is synchronous, the bitmap
will be adopted by the receiving end, and disowned by the sender nicely
without any accounting effort like we've had to do in the past.

Use this in NotificationServer to allow sending arbitrary bitmaps as
icons instead of paths-to-icons.

Libraries/LibGfx/ShareableBitmap.h

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <AK/RefPtr.h>
+#include <LibGfx/Bitmap.h>
+#include <LibGfx/Size.h>
+
+namespace Gfx {
+
+class ShareableBitmap {
+public:
+    ShareableBitmap() {}
+    explicit ShareableBitmap(const Gfx::Bitmap&);
+
+    bool is_valid() const { return m_bitmap; }
+
+    i32 shbuf_id() const { return m_bitmap ? m_bitmap->shbuf_id() : -1; }
+
+    const Bitmap* bitmap() const { return m_bitmap; }
+    Bitmap* bitmap() { return m_bitmap; }
+
+    Size size() const { return m_bitmap ? m_bitmap->size() : Size(); }
+    Rect rect() const { return m_bitmap ? m_bitmap->rect() : Rect(); }
+
+    int width() const { return size().width(); }
+    int height() const { return size().height(); }
+
+private:
+    RefPtr<Bitmap> m_bitmap;
+};
+
+}
+
+namespace IPC {
+bool decode(Decoder&, Gfx::ShareableBitmap&);
+}