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.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/395
Previously gnome-shell listened on the Xft Xsettings via GTK+s
GtkSettings to get the font DPI setting. The Xsetting might not
be what we want, and we should not rely on Xsettings when we don't need
to, so lets manage it ourself.
https://bugzilla.gnome.org/show_bug.cgi?id=765011
Introduce MetaSettings and add the settings managed by MetaBackend into
the new object. These settings include: experimental-features and UI
scaling factor.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
