Make meson link libmutter using -fvisibility=hidden, and introduce META_EXPORT
and META_EXPORT_TEST defines to mark a symbols as visible.
The TEST version is meant to be used to flag symbols that are only used
internally by mutter tests, but that should not be considered public API.

This allows us to be more precise in selecting what is exported and what is
not, without the need of a version-script file that would be more complicated
to maintain.

!395