WindowServer: Implement simple window shadows

This implements simple window shadows around most windows, including
tooltips. Because this method uses a bitmap for the shadow bits,
it is limited to rectangular window frames. For non-rectangular
window frames we'll need to implement a more sophisticated algorithm.

Userland/Libraries/LibGfx/SystemTheme.cpp

@@ -119,6 +119,7 @@ Core::AnonymousBuffer load_system_theme(const String& path)
     } while (0)
 
     DO_PATH(TitleButtonIcons);
+    DO_PATH(WindowShadow);
 
     return buffer;
 }