MetaWaylandPointer: Put client resources in its own struct

Instead of moving around all the bound pointer resources for a client
when changing focus, keep all the resources bound by a client in a per
client struct, and track the focus by having a pointer to the current
active pointer client struct instance.

This will simplify having wl_pointer extensinos sharing the pointer
focus of the wl_pointer by only having to add them to the pointer
client.

https://bugzilla.gnome.org/show_bug.cgi?id=744104

src/wayland/meta-wayland-types.h

@@ -43,4 +43,6 @@ typedef struct _MetaWaylandOutput MetaWaylandOutput;
 
 typedef struct _MetaWaylandSerial MetaWaylandSerial;
 
+typedef struct _MetaWaylandPointerClient MetaWaylandPointerClient;
+
 #endif