i3bar: quick & dirty systray implementation

Works correctly only with exactly one dock client on exactly one output. Maybe
not even then. You have been warned. Proof-of-concept code ;).

i3bar/src/outputs.c

@@ -157,6 +157,7 @@ static int outputs_start_map_cb(void *params_) {
         new_output->ws = 0,
         memset(&new_output->rect, 0, sizeof(rect));
         new_output->bar = XCB_NONE;
+        new_output->traypx = 0;
 
         new_output->workspaces = malloc(sizeof(struct ws_head));
         TAILQ_INIT(new_output->workspaces);