next_on_output does not cycle through all workspaces

## I'm submitting a…

[x] Bug

[ ] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

## Current Behavior
I have four workspaces, with the following names:
- 1
- 2b
- 2
- 3

They are ordered in the same way as the list above. If I have my focus on workspace "1", and I use the keybinding for "next_on_output", it goes from "1" to "2b", and then to "3". If I have my focus on workspace "3", and I use they keybinding for "prev_on_output", it goes from "3" to "2", and then to "1".

## Expected Behavior
I expect the workspaces to be ordered as follows:
- 1
- 2
- 2b
- 3

More importantly, when using the keybindings for "next_on_output" while focussing workspace "1", I expect to go to "2", then to "2b", and finally to "3". Similar for "prev_on_output".

## Reproduction Instructions
Open four workspaces with the following names:
- 1
- 2b
- 2
- 3

Focus workspace "1". Use your keybinding for "next_on_output" and observe how it jumps over workspace "2".

## Environment
Output of `i3 --moreversion 2>&-`:

Binary i3 version:  4.15 (2018-03-10) © 2009 Michael Stapelberg and contributors

(Getting version from running i3, press ctrl-c to abort…)

Running i3 version: 4.15 (2018-03-10) (pid 614)

Loaded i3 config: /home/[REDACTED]/.i3/config (Last modified: Wed 15 Aug 2018 01:25:34 PM CEST, 5839947 seconds ago)


The i3 binary you just called: /usr/bin/i3

The i3 binary you are running: i3

I don't think either config or logfile is needed, but I'm happy to provide more information if needed. Here's a link to my config:
https://github.com/basilschneider/dotfiles/blob/master/.i3/config

- Linux Distribution & Version: ArchLinux (Linux [REDACTED] 4.18.12-arch1-1-ARCH #1 SMP PREEMPT Thu Oct 4 01:01:27 UTC 2018 x86_64 GNU/Linux)

- Are you using a compositor (e.g., xcompmgr or compton): No