Initial import. Most things seem working.

This includes an abortive attempt to do a gtk4 dialog (which I don't
think is possible, as gtk4 doesn't allow embedding toplevels anymore),
and an iced dialog, which I just never started writing.

Cargo.toml

@@ -0,0 +1,14 @@
+[workspace]
+members = [
+    "async-xcb",
+    "auth",
+    "command",
+    "locker",
+    "dbus-service",
+    "dialog-gtk3",
+#    "dialog-gtk4",
+#    "dialog-iced",
+    "util",
+    "systemd",
+    "xcb-xembed",
+]