WindowServer: Support PNG wallpapers.

Fix up /bin/pape so it tells the WindowServer which wallpaper file to use.

LibGUI/GDesktop.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <AK/AKString.h>
+#include <SharedGraphics/Rect.h>
+
+class GDesktop {
+public:
+    static GDesktop& the();
+
+    String wallpaper() const;
+    bool set_wallpaper(const String& path);
+
+private:
+    GDesktop();
+
+    Rect m_rect;
+};