Qt6 frontend

WIP - do not commit

Basically a copy of qt5 to qt6

TODO remove all the \since since they don't make sense anymore

Changes:
 * TODO
6 jobs for qt6 in 2 minutes and 56 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #3326140
build

00:02:56

passed #3326145
build_android

00:00:32

passed #3326141
build_clang_libcpp

00:02:33

passed #3326144
build_clazy

00:02:21

passed #3326143
build_mingw64_fedora

00:02:04

passed #3326142
build_ubuntu_16_04

00:01:31