This commit makes it possible to configure logical monitor scale also
when running on top of an X11 server using Xrandr. An extra property
'requires-globla-scale' is added to the D-Bus API is added to instruct
a configuration application to only allow setting a global logical
monitor scale.
This is needed to let gsd-xsettings use the configured state to set a
XSettings state that respects the explicit monitor configuration.
https://bugzilla.gnome.org/show_bug.cgi?id=777732