[libkwineffects] Add windowId to EffectWindow

My use case is integrating with a system that tracks window properties based on the windowId. This allows the effect to do window-specific things.

effects.cpp

@@ -1833,6 +1833,7 @@ TOPLEVEL_HELPER(KWaylandServer::SurfaceInterface *, surface, surface)
 TOPLEVEL_HELPER(bool, isPopupWindow, isPopupWindow)
 TOPLEVEL_HELPER(bool, isOutline, isOutline)
 TOPLEVEL_HELPER(pid_t, pid, pid)
+TOPLEVEL_HELPER(qlonglong, windowId, windowId)
 
 #undef TOPLEVEL_HELPER
 

effects.h

@@ -454,6 +454,7 @@ public:
     bool isX11Client() const override;
 
     pid_t pid() const override;
+    qlonglong windowId() const override;
 
     QRect decorationInnerRect() const override;
     QByteArray readProperty(long atom, long type, int format) const override;

libkwineffects/kwineffects.h

@@ -2439,6 +2439,11 @@ public:
      */
     virtual pid_t pid() const = 0;
 
+    /**
+     * @since 5.21
+     */
+    virtual qlonglong windowId() const = 0;
+
     /**
      * Can be used to by effects to store arbitrary data in the EffectWindow.
      *