WindowServer+LibGUI: Pass window icons as shared buffers rather than paths.

Now that we support more than 2 clients per shared buffer, we can use them
for window icons. I didn't do that previously since it would have made the
Taskbar process unable to access the icons.

This opens up some nice possibilities for programmatically generated icons.

Libraries/LibGUI/GEvent.h

@@ -40,6 +40,7 @@ public:
         WM_WindowStateChanged,
         WM_WindowRectChanged,
         WM_WindowIconChanged,
+        WM_WindowIconBitmapChanged,
         __End_WM_Events,
     };
 
@@ -133,6 +134,23 @@ private:
     String m_icon_path;
 };
 
+class GWMWindowIconBitmapChangedEvent : public GWMEvent {
+public:
+    GWMWindowIconBitmapChangedEvent(int client_id, int window_id, int icon_buffer_id, const Size& icon_size)
+        : GWMEvent(GEvent::Type::WM_WindowIconBitmapChanged, client_id, window_id)
+        , m_icon_buffer_id(icon_buffer_id)
+        , m_icon_size(icon_size)
+    {
+    }
+
+    int icon_buffer_id() const { return m_icon_buffer_id; }
+    const Size& icon_size() const { return m_icon_size; }
+
+private:
+    int m_icon_buffer_id;
+    Size m_icon_size;
+};
+
 class GMultiPaintEvent final : public GEvent {
 public:
     explicit GMultiPaintEvent(const Vector<Rect, 32>& rects, const Size& window_size)