Add back the WindowsGoBelow panel behavior

In relation to patches in https://invent.kde.org/frameworks/plasma-framework/-/commit/d0c9c659db4b35f98b7d9a3b2f8afb9fe5d73f34 and https://invent.kde.org/frameworks/plasma-framework/-/commit/94a0495f137d966442b13d05dbf8c215260b35f5, I noticed that the WindowsGoBelow behavior is missing in the new implementation. Before those two patches, when Dock type was used, WindowsGoBelow was also set. After the aforementioned patches, Dock type is still set, but WindowsGoBelow was removed. While I could not see any regression caused by the lack of the behavior, and I do not see any difference after adding it, I still thought I should propose this to your attention. You can also discard this patch if you think it is better.