"panel-background.svg" should be updated to work on Plasma 5

[skylee03]

According to the official document, since Plasma 5, all "[prefix-]" elements should include all potential borders. However, currently we only have "north-bottom", "east-left", "south-top", and "west-right", with other borders missing, which would cause the panel background to fallback to the default theme in KDE Plasma.

[skylee03]

"QTStep Classic" themes do include all potential borders, and they work well, while "QTStep" themes do not.

[makubo]

I've got the same issue after update plasma from 5.21.5 to 5.22.5.
Before all was fine. (Gentoo)