Blender Git Loki

Fix T43128: Headerless panels are reorderable on Windows

Seems like a weird issue, but to sort panels "qsort" is used, which works slightly different on Windows. So all I had
to do was cleaning up the logic in find_highest_panel so that headerless panels are sorted, but that it absolutly not
allows headerless panels to be above normal panels.
I made sure it works fine on Linux as well.