Added 'focus_on_window_activation' directive

When a window receives a _NET_ACTIVE_WINDOW message, it can steal the focus. This may not be preferable to all users.
With this directive, the user can choose from one of the following:
1) 'smart' - focus the container if its workspace is visible, otherwise set the urgency flag (default)
2) 'urgent' - always set the urgency flag, do not steal focus
3) 'focus' - always switch focus, never set the urgency hint
4) 'none' - ignore the request entirely (do not switch focus, nor set the urgency hint)

fixes #1426

parser-specs/config.spec

@@ -38,6 +38,7 @@ state INITIAL:
   'workspace_auto_back_and_forth'          -> WORKSPACE_BACK_AND_FORTH
   'fake_outputs', 'fake-outputs'           -> FAKE_OUTPUTS
   'force_display_urgency_hint'             -> FORCE_DISPLAY_URGENCY_HINT
+  'focus_on_window_activation'             -> FOCUS_ON_WINDOW_ACTIVATION
   'workspace'                              -> WORKSPACE
   'ipc_socket', 'ipc-socket'               -> IPC_SOCKET
   'restart_state'                          -> RESTART_STATE
@@ -210,6 +211,11 @@ state FORCE_DISPLAY_URGENCY_HINT_MS:
   end
       -> call cfg_force_display_urgency_hint(&duration_ms)
 
+# focus_on_window_activation <smart|urgent|focus|none>
+state FOCUS_ON_WINDOW_ACTIVATION:
+  mode = word
+      -> call cfg_focus_on_window_activation($mode)
+
 # workspace <workspace> output <output>
 state WORKSPACE:
   workspace = word