window.c: Handle multiple keyboard foci

Keep a count of number of wl_keyboard focus we have instead of
tracking the more recent wl_keyboard.
diff --git a/clients/window.h b/clients/window.h
index da18932..779bc64 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -205,6 +205,9 @@
 struct window *
 window_create_custom(struct display *display);
 
+int
+window_has_focus(struct window *window);
+
 typedef void (*menu_func_t)(struct window *window, int index, void *data);
 
 void