Grouped Window List loses taskbar window order on certain events (connecting/disconnecting external monitor, others)

 * Cinnamon version 4.8.6
 * Distribution - (Mint 20.1)
 * Intel HD Graphics 620
 * 64 bit

**Issue**
The Grouped Window List fairly frequently loses track of window order
and jumbles the order of the applications in the task bar.
In Linux Mint 20.1, this has been happening much more frequently, (see for example #9802).
Closing a window is not the only cause of this, however.
One reliable way to reproduce this for me is to connect or disconnect
my external monitor to the HDMI on my laptop.
For me, connecting my HDMI monitor has reliably caused this to happen for two years or more. (18.x, 19.x, 20.x)
Since I have upgraded to 20.1, I have also noticed that this is happening much more frequently,
and seemingly more than just with closing windows.

**Steps to reproduce**
1. Use Grouped Window List (see config below)
2. Open several application windows
3. Connect an HDMI display
4. Note that order of windows is jumbled

Before connecting external monitor:
![taskbar_before_external](https://user-images.githubusercontent.com/3146028/110403080-7c85a280-804a-11eb-9417-093a03d899bc.png)
After connection external monitor:
![taskbar_after_external](https://user-images.githubusercontent.com/3146028/110403086-814a5680-804a-11eb-8bdb-b8a1be89348b.png)


**Expected behaviour**
Windows that are open would retain the same order in the Taskbar/Panel through normal use.
Moving open window icons with the mouse would change the order,
but other than that, the order would be maintained.

**Other information**
The pull request #9899 is supposed to fix the issue for the order being lost when a window is closed,
but I am not sure if it will do anything about this more general case of jumbling.
I will be filing a very similar bug about the order within window groups being jumbled in the same situations.

**Configuration info, if applicable**
[groupedwindowlist.json.txt](https://github.com/linuxmint/cinnamon/files/6104986/groupedwindowlist.json.txt)

Start by reproducing the order change with the Grouped Window List while connecting or disconnecting an external HDMI display, then compare the behavior with pull request #9899. The work is done when open windows retain their taskbar order through these events, while deliberate mouse reordering still changes it.