gwl: Add overflow support through scroll

[anaximeno]

closes #11540
solves https://github.com/orgs/linuxmint/discussions/82

Example of how should work

2026-01-21.17-26-02.mp4

[Initu-Castilhos]

This feature is great, it's similar to dash to dock and dash to panel. Hopefully it will be implemented in the stable version.

[JosephMcc]

Didn't actually try it yet, but it looks pretty slick. Wonder if you could use vfade when the slider button shows up as a bit more of a visual cue.

[anaximeno]

Wonder if you could use vfade when the slider button shows up as a bit more of a visual cue.

I'll give it a try and see if I can get it to work with it.

[anaximeno]

@JosephMcc I've updated the implementation to use the native St.ScrollView instead of a direct container translation to handle the scroll, which goes with how scroll is being implemented accross Cinnamon. I wanted to use it initially as well but was having some issues with the scroll not taking effect in the gwl and instead the same stretch behavior was persisting, however I found the reason for it and it seems it was that since there was no overflow method before, a change was introduced to set the min_size of the AppGroup.actor around 1 (which meant instead of the container knowing what's the proper size to represent the full content it was considering that it should just stretch almost indefinetely, so I reverted that behavior in https://github.com/linuxmint/cinnamon/pull/13352/changes#diff-ff87e15fbe5d4a462aeef966bf403d5543648b31289a55f4b835cdcfcb32f362.

Thanks to that and getting St.ScrollView to work, I can also use the vfade/hfade in the edges instead of the slider buttons (as you suggested), I've updated the video demonstration above to represent that.

In general I think this PR is ready for testing and consideration, and I'll be continuing to test and it sending adjustments if necessary until the next merge window.

Regards.