wayland: Make it possible to trigger popups through pointer/keyboard/touch

Right now we just check the pointer serial, so the popup will be
immediately dismissed if the client passes a serial corresponding to
another input device.

Abstract this a bit further and add a meta_wayland_seat_can_popup() call
that will check the serial all input devices. This makes it possible to
trigger menus through touch or keyboard devices.

https://bugzilla.gnome.org/show_bug.cgi?id=756296

src/wayland/meta-wayland-keyboard.c

@@ -673,3 +673,10 @@ meta_wayland_keyboard_create_new_resource (MetaWaylandKeyboard *keyboard,
       wl_list_insert (&keyboard->resource_list, wl_resource_get_link (cr));
     }
 }
+
+gboolean
+meta_wayland_keyboard_can_popup (MetaWaylandKeyboard *keyboard,
+                                 uint32_t             serial)
+{
+  return keyboard->key_serial == serial;
+}