Add option for Qt6 mode in meson based builds. We should probably make Qt6 directly the default in master, to match autotools based build after #43332.
This depends on #43362 (minimum meson version 0.57.0)
meson-winbuild.sh needs to be adjusted too. Things will look better after #44579 (so it's dependency for this ticket), but this patch needs adjustment too.
Add option for Qt6 mode in meson based builds. We should probably make Qt6 directly the default in master, to match autotools based build after #43332.
This depends on #43362 (minimum meson version 0.57.0)