window: Enable pinging on Wayland windows

I implemented pinging, but never actually enabled the feature
properly on Wayland surfaces by setting the net_wm_ping hint to
TRUE, causing the fallback path to always be hit.

Rename net_wm_ping to can_ping so it doesn't take on an
implementation-specific meaning, and set it for all Wayland windows.

src/core/display.c

@@ -4980,7 +4980,7 @@ meta_display_ping_window (MetaWindow        *window,
       return;
     }
 
-  if (!window->net_wm_ping)
+  if (!window->can_ping)
     {
       if (ping_reply_func)
         (* ping_reply_func) (window, timestamp, user_data);