meson: Change x11 and introspection option to feature

Meson has built-in support for yes/no/auto options, they are called "feature": https://mesonbuild.com/Build-options.html#features.

They have the main advantage of using the --auto-features=disabled global switch that turn all auto features to disabled (likewise for enabling them all). Distro like that because it means they don't rely on auto detections during package build.

This MR also had the advantage of being more consistent with other projects that have the same introspection option. When at-spi-core is a subproject of another project (e.g. gtk) it means that enabling/disabling introspection on the main project will automatically do the same on the subproject. That's what the yield: true line does.

This was previously submitted in atk!47.