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

include/config.h

@@ -167,6 +167,18 @@ struct Config {
      * flag can be delayed using an urgency timer. */
     float workspace_urgency_timer;
 
+    /** Behavior when a window sends a NET_ACTIVE_WINDOW message. */
+    enum {
+        /* Focus if the target workspace is visible, set urgency hint otherwise. */
+        FOWA_SMART,
+        /* Always set the urgency hint. */
+        FOWA_URGENT,
+        /* Always focus the window. */
+        FOWA_FOCUS,
+        /* Ignore the request (no focus, no urgency hint). */
+        FOWA_NONE
+    } focus_on_window_activation;
+
     /** The default border style for new windows. */
     border_style_t default_border;