x11: Add "source_xwindow" parameter to MetaPropValue

It will become necessary to track properties and changes from frame windows,
and it will be more convenient to have this managed by the common property
tracking mechanisms.

Add this source_xwindow parameter so property handler functions can check
whether the property belonged to the client Window or the frame Window.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2175>

src/x11/xprops.c

@@ -958,6 +958,8 @@ meta_prop_get_values (MetaX11Display *x11_display,
           goto next;
         }
 
+      values[i].source_xwindow = xwindow;
+
       switch (values[i].type)
         {
         case META_PROP_VALUE_INVALID:

src/x11/xprops.h

@@ -132,6 +132,7 @@ typedef struct
   MetaPropValueType type;
   Atom atom;
   Atom required_type; /* autofilled if None */
+  Window source_xwindow;
 
   union
   {