Partially revert 3af1853260 to fix use-after-free

After upgrading to the latest release of Waybar the bar will crash
whenever I close the laptop lid. After some debugging I believe it is
because the watching added by watch_name is not being correctly canceled
using unwatch_name. After the Tray object and Host object are destroyed,
additional callbacks will become use-after-free.

Looks like commit 3af1853260 removed the
unwatch_name. I'm not sure why it did that, but it seemed dangerous.

Additionally, bus_name_id_ is created by own_name. According to that
function's documentation, the correct inverse operation is unown_name.

src/modules/sni/watcher.cpp

@@ -14,6 +14,10 @@ Watcher::Watcher()
       watcher_(sn_watcher_skeleton_new()) {}
 
 Watcher::~Watcher() {
+  if (hosts_ != nullptr) {
+    g_slist_free_full(hosts_, gfWatchFree);
+    hosts_ = nullptr;
+  }
   if (items_ != nullptr) {
     g_slist_free_full(items_, gfWatchFree);
     items_ = nullptr;