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/host.cpp

@@ -19,9 +19,13 @@ Host::Host(const std::size_t id, const Json::Value& config, const Bar& bar,
 
 Host::~Host() {
   if (bus_name_id_ > 0) {
-    Gio::DBus::unwatch_name(bus_name_id_);
+    Gio::DBus::unown_name(bus_name_id_);
     bus_name_id_ = 0;
   }
+  if (watcher_id_ > 0) {
+    Gio::DBus::unwatch_name(watcher_id_);
+    watcher_id_ = 0;
+  }
   g_cancellable_cancel(cancellable_);
   g_clear_object(&cancellable_);
   g_clear_object(&watcher_);

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;