Handle better when the grouped window list is full of items

[anaximeno]

I am proposing a fix for linuxmint/cinnamon#11540

This video is one of my main references here.

The Issue

When many applications are open on the grouped-window-list applet or the panel is full of items, the icons of the grouped window list will start to look cropped:

The Solution(s)

These seem to be the more adequate solutions for this problem:

1. [ref] Make the section of grouped-window-list applet scrollable in the same direction as the panel
2. [ref] Make the section of grouped-window-list applet scrollable in the opposite direction of the panel
3. [ref] Hide the app icons that would make the app list exceed the available space and show a symbol/icon which would show the remaining app icons when clicked.

Another solution I was thinking about is to make the entire panel section scrollable instead of only the grouped window list. The reason for that is that some applets other than app lists also take a lot of space from the panel so it isn't difficult to have the panel full even when not having a app list on that panel.

These three colored sections compose the panel sections:

The idea would be to make one (or both) of the sections scrollable whenever one of the sections touches another if there's no more space available to grow in size.

My concern with this particular solution is that there are some items of the panel you wouldn't want to be hidden (like the menu button) whenever they are present in the panel, but if the section is completely scrollable (and not only the grouped window list or app list), there's a chance you may hide it when scrolling the section of the panel.