Add con_exists function

Checks the all_cons queue and returns true if a given con is found.

src/con.c

@@ -597,6 +597,15 @@ Con *con_by_con_id(long target) {
     return NULL;
 }
 
+/*
+ * Returns true if the given container (still) exists.
+ * This can be used, e.g., to make sure a container hasn't been closed in the meantime.
+ *
+ */
+bool con_exists(Con *con) {
+    return con_by_con_id((long)con) != NULL;
+}
+
 /*
  * Returns the container with the given frame ID or NULL if no such container
  * exists.