fix(river): hide vacant tags on initial startup

Before this, vacant tags would show with `hide-vacant` set on initial
startup, because we receive initial tag events from River before we show
the bar. In that case, we won't call `set_visible(false)` on the
respective buttons because they're not shown yet. This registers the
output status listener after we show the bar so we won't miss any
events.

include/modules/river/tags.hpp

@@ -21,6 +21,7 @@ class Tags : public waybar::AModule {
   void handle_view_tags(struct wl_array *tags);
   void handle_urgent_tags(uint32_t tags);
 
+  void handle_show();
   void handle_primary_clicked(uint32_t tag);
   bool handle_button_press(GdkEventButton *event_button, uint32_t tag);