On startup and before we connected to a tablet, let's display a missing tablet
icon. Taken from the tablet icon, combined with the X from one of the network
icons in the Adwaita theme.
Once we get a sync notification, we just hide that one and done.
This is a plain subtree merge without any changes to the tuhigui tree itself.
As we'll keep the distinction between the DBus server and the GTK UI, the
directory layout will reflect that.