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