fix: add missing c-tor for nesting Variants (#467)

This allows nesting variants, i.e. allows a variant to contain another variant as its value. Until now, there was no such possibility, since the default generated copy constructor would be invoked which would create a copy of source variant instead of embed the source variant as a value in the destination variant. The default generated copy constructor is kept, for it makes sense too, but a new tag-based overload is added for embedding the source variant into the destination variant.

tests/integrationtests/integrationtests-proxy.h

@@ -225,6 +225,16 @@ public:
         m_proxy.setProperty("action").onInterface(INTERFACE_NAME).toValue(value);
     }
 
+    sdbus::Variant actionVariant()
+    {
+        return m_proxy.getProperty("actionVariant").onInterface(INTERFACE_NAME).get<sdbus::Variant>();
+    }
+
+    void actionVariant(const sdbus::Variant& value)
+    {
+        m_proxy.setProperty("actionVariant").onInterface(INTERFACE_NAME).toValue({value, sdbus::embed_variant});
+    }
+
     bool blocking()
     {
         return m_proxy.getProperty("blocking").onInterface(INTERFACE_NAME).get<bool>();