meson: Reorganize meson options a bit

Enable core gstreamer subprojects bad, ugly, libav, ges, devtools by
default. Otherwise if, say, you do, -Dgst-plugins-bad:dtls=enabled and
openssl is not found, meson will disable the entire subproject. You
have to pass -Dbad=enabled -Dgst-plugins-bad:dtls=enabled to get the
expected behaviour.

Also move options that are not for selection subprojects out of the
section, add a qt6 option, and improve the description for some
options.

Part-of: <!3708>