Add coordinates relative to the current output in i3bar click events

Currently i3bar click events provide x and y coordinates relative to all monitors.
I've added coordinates relative to the current output.

+-----------+-----------+
|           |   i3bar   |
|           +-----------+
|   HDMI-0  |    DP-0   |
| 1920x1080 | 2560x1080 |
+-----------+-----------+

When you click in the top right corner of the DP-0,
i3bar will provide something like this:

{
  "x": 4480,
  "y": 10,
  "output_x": 2560,
  "output_y": 10,
}

This is useful for creating a rofi menu or something else.
rofi -show run -location 1 -xoffset ${I3_OUTPUT_X} -yoffset ${I3_OUTPUT_Y}

i3bar/include/child.h

@@ -85,4 +85,4 @@ bool child_want_click_events(void);
  * Generates a click event, if enabled.
  *
  */
-void send_block_clicked(int button, const char *name, const char *instance, int x, int y, int x_rel, int y_rel, int width, int height, int mods);
+void send_block_clicked(int button, const char *name, const char *instance, int x, int y, int x_rel, int y_rel, int out_x, int out_y, int width, int height, int mods);