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/window-private.h

@@ -238,7 +238,7 @@ struct _MetaWindow
   /* These are the flags from WM_PROTOCOLS */
   guint take_focus : 1;
   guint delete_window : 1;
-  guint net_wm_ping : 1;
+  guint can_ping : 1;
   /* Globally active / No input */
   guint input : 1;