feat: use singleton connection to systemstats

it has been found in #174, that if there are multiple instances of
widget, it keeps subscribing on all sources, because of how dbus
interface works.

In order to hande it, all subscriptions are now kept from singleton with
tracking active clients

sources/awesome-widget/plugin/formatters/awpluginformatter.h

@@ -32,7 +32,7 @@ public:
     [[nodiscard]] virtual QString format(const QVariant &_value, const QString &_key,
                                          const AWPluginFormatSettings &_settings) const
         = 0;
-    virtual void load(){};
+    virtual void load() {};
 };