Q:

How can I get Icon only "Grouped Modern Window list" on Xubuntu, that I can "pin" in panel too?

I want to use Grouped window list (Just like in Windows 7 Taskbar).
I am using Xubuntu 18.04. 

A:

Right-click on a blank space in the panel
Open Panel > Panel Preferences
Click on the Items tab
Click on Window Buttons and then on the cog-wheel/gear icon
Make your choices there; the change to Window grouping will be visible in the panel immediately 
Close the Window Buttons window and the Panel window

As the image shows, there are three Firefox windows which are listed when you left-click on the Firefox icon. You can right-click on each item for more options. Alternatively, you can right-click on the Firefox icon to see the list as well as available options. 

