Add Xsettings support, for setting scaling related settings

This allows for most GTK and Qt apps to be scaled properly.
In the case of mixed DPI, it will default to using the smallest monitor scale.

src/server/dispatch.rs

@@ -1272,6 +1272,7 @@ impl<C: XConnection> GlobalDispatch<WlOutput, Global> for ServerState<C> {
             Output::new(client, server).into()
         });
         state.output_keys.insert(key, ());
+        state.output_scales_updated = true;
     }
 }
 global_dispatch_with_events!(WlDrmServer, WlDrmClient);