Make the manager listen for changes on new window

When mapping a new window, listen for property notify events.

spec/uh/wm/manager_spec.rb

@@ -133,6 +133,8 @@ module Uh
       end
 
       describe '#map' do
+        let(:display) { instance_spy Display }
+
         it 'registers a new client wrapping the given window' do
           manager.map window
           expect(manager.clients[0])
@@ -159,6 +161,13 @@ module Uh
           end
           manager.map window
         end
+
+        it 'listens for property notify events on given window' do
+          expect(display)
+            .to receive(:listen_events)
+            .with window, Events::PROPERTY_CHANGE_MASK
+          manager.map window
+        end
       end
 
       describe '#unmap' do