Introduced by qxl, add these properties as a generic way to tell a
display manager about the GUI layout.

Also add the hotplug_mode_update_property which advises display managers to
reread the mode list on a hotplug event.
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>