Implement drag cancel for resizing tiled windows

Long description:

Hi, I am a new developer and this is my first code submission to a
project. If I do anything wrong, please let me know.

The patches I've submitted implement #1074 "Add ability to escape
out of a mouse-resize operation".

To apply this patch, use:
curl http://cr.i3wm.org/patch/236/raw.patch | git am

@@ -106,12 +106,16 @@ int resize_graphical_handler(Con *first, Con *second, orientation_t orientation,

     const struct callback_params params = { orientation, output, helpwin, &new_position };

-    drag_pointer(NULL, event, grabwin, BORDER_TOP, 0, resize_callback, &params);

+    int drag_result = drag_pointer(NULL, event, grabwin, BORDER_TOP, 0, resize_callback, &params);

     xcb_destroy_window(conn, helpwin);

     xcb_destroy_window(conn, grabwin);

     xcb_flush(conn);

+    // User cancelled the drag so no action should be taken.

+    if (drag_result == 1)

+        return 0;

+

     int pixels;

     if (orientation == HORIZ)

         pixels = (new_position - event->root_x);