Quit on `mod1+q' key binding

* Loop our runner until it is "stopped";
* Use the event dispatcher to stop the runner on `mod1+q' key press;
* Tell the manager to grab the `q' key.

features/steps/x_steps.rb

@@ -1,3 +1,11 @@
+def x_key key
+  fail "cannot simulate X key `#{key}'" unless system "xdotool key #{key}"
+end
+
+When /^I press the default quit key binding$/ do
+  x_key 'alt+q'
+end
+
 Then /^it must connect to X display$/ do
   uhwm_wait_output 'Connected to'
   expect(`sockstat -u`.lines.grep /\s+ruby.+\s+#{@process.pid}/)