Add META_GRAB_OP_WAYLAND_CLIENT

Which is used for Wayland popup grabs.

The issue here is that we don't want the code that raises or focuses
windows based on mouse ops to run while a client has a grab.

We still keep the "old" grab infrastructure in place for now, but
ideally we'd replace it eventually with a better grab-op infrastructure.

src/meta/common.h

@@ -228,7 +228,10 @@ typedef enum
   META_GRAB_OP_CLICKING_UNSTICK,
 
   /* Special grab op when the compositor asked for a grab */
-  META_GRAB_OP_COMPOSITOR
+  META_GRAB_OP_COMPOSITOR,
+
+  /* For when a client takes a popup grab */
+  META_GRAB_OP_WAYLAND_CLIENT,
 } MetaGrabOp;
 
 /**