cinnamon: menu -> add to panel can result in new panel launcher applets being added ad infinitum

[smurphos]

I have reported this before - the issue has been apparent since Cinnamon 4.0.x.

linuxmint/cinnamon#8390
linuxmint/mint19.2-beta#64

To reproduce.

1. Remove GWL from the panel
2. From the menu, right click an application e.g. Celluloid, and choose add to panel. Observe a panel launcher is created with the 3 default launchers but not Celluloid.
3. Try again, observe a 2nd panel launcher with the 3 default launchers is created, but no Celluloid.
4. Repeat until bored.. ;-)

A Cinnamon restart between steps 2 and 3 fixes the issue.

Alternative workflow

1. Remove GWL from the panel
2. Add a panel-launcher applet
3. From the menu, right click an application e.g. Celluloid, and choose add to panel. Observe a 2nd panel launcher is created with the 3 default launchers but not Celluloid.
4. Try again, observe a 3nd panel launcher with the 3 default launchers is created, but no Celluloid.
5. Repeat until bored.. ;-)

A Cinnamon restart between steps 2 and 3 fixes the issue.

In reverse

1. Switch to traditional layout.
2. Remove the panel launcher applet
3. Add the GWL applet
4. From the menu, right click an application e.g. Celluloid, and choose add to panel. Observe a panel launcher is created with the 3 default launchers and Celluloid. Further attempts to add to panel will add additional Celluliod instances to the panel launcher.

Restarting Cinnamon in between steps 3 + 4 avoids this issue., and add to panel pins apps in the GWL as expected.