Add relative coordinates in JSON for i3bar click events (fixes #2767)

Add support for relative coordinates in i3bar click events

Rename {x,y}_rel to relative_{x,y}

Update i3bar-protocol doc to mention the added fields in click events

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);
+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);