Improve UAT on layout

* Move cucumber features regarding the layout protocol in
  layout/protocol;
* Add new cucumber features to test uh-layout integration in uh-wm, at
  least simple mapping and focus behavior;
* Add new acceptance testing helpers

features/layout/manage.feature

@@ -1,16 +1,12 @@
 Feature: layout client management
 
-  Scenario: sends the #<< message when telling the layout to manage a client
-    Given a file named layout.rb with:
-      """
-      class Layout
-        def register *_; end
+  Background:
+    Given uhwm is running
 
-        def << client
-          puts client
-        end
-      end
-      """
-    And uhwm is running with options -v -r./layout -l Layout
+  Scenario: maps new client window
     When a window requests to be mapped
-    Then the output must contain the window name
+    Then the window must be mapped
+
+  Scenario: focuses new client window
+    When a window requests to be mapped
+    Then the window must be focused

features/layout/protocol.feature

@@ -0,0 +1,24 @@
+Feature: layout protocol
+
+  Background:
+    Given a file named layout.rb with:
+      """
+      class Layout
+        def register display
+          puts display
+        end
+
+        def << client
+          puts client
+        end
+      end
+      """
+
+  Scenario: tells the layout to register with #register message
+    When I run uhwm with option -r./layout -l Layout
+    Then the output must contain current display
+
+  Scenario: tells the layout to manage a client with #<< message
+    Given uhwm is running with options -v -r./layout -l Layout
+    When a window requests to be mapped
+    Then the output must contain the window name

features/layout/registration.feature

@@ -1,13 +0,0 @@
-Feature: layout registration
-
-  Scenario: sends the #register message to the layout with the display
-    Given a file named layout.rb with:
-      """
-      class Layout
-        def register display
-          puts display
-        end
-      end
-      """
-    When I run uhwm with option -r./layout -l Layout
-    Then the output must contain current display