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.h

@@ -144,6 +144,7 @@ enum class MetricRole {
 enum class PathRole {
     NoRole,
     TitleButtonIcons,
+    WindowShadow,
     __Count,
 };