Implement `kill' action keyword

When invoked, will kill the client returned by
`layout.current_client`. ICCCM WM_DELETE_WINDOW is used instead if the
window reports to support this protocol.

features/steps/x_steps.rb

@@ -1,3 +1,10 @@
+Given /^an ICCCM compliant window is mapped$/ do
+  icccm_window_start
+  timeout_until 'window not mapped after %d seconds' do
+    x_window_map_state(icccm_window_name) == 'IsViewable'
+  end
+end
+
 Given /^a(?:\s(\w+))? window is mapped$/ do |ident|
   x_client(ident).map.sync
   timeout_until 'window not mapped after %d seconds' do
@@ -42,6 +49,10 @@ Then /^the(?:\s(\w+))? window must be mapped$/ do |ident|
   end
 end
 
+Then /^the ICCCM window must be unmapped by the manager$/ do
+  uhwm_wait_output /unmanag.+#{icccm_window_name}/i
+end
+
 Then /^the window must be focused$/ do
   timeout_until 'window not focused after %d seconds' do
     x_focused_window_id == x_client.window_id