[qtquickrenderersettings] Support vulkan backend selection
Qt6 requires explicit use of the enum types for selecting the RHI backend, so kcfg is updated to not use a lazy string trick, but stay in line with the kcm config file.
Qt6 requires explicit use of the enum types for selecting the RHI backend, so kcfg is updated to not use a lazy string trick, but stay in line with the kcm config file.