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/DBusPropertiesTests.cpp

@@ -81,3 +81,12 @@ TYPED_TEST(SdbusTestObject, CanAccessAssociatedPropertySetMessageInPropertySetHa
     ASSERT_THAT(this->m_adaptor->m_propertySetMsg, NotNull());
     ASSERT_THAT(this->m_adaptor->m_propertySetSender, Not(IsEmpty()));
 }
+
+TYPED_TEST(SdbusTestObject, WritesAndReadsReadWriteVariantPropertySuccessfully)
+{
+    sdbus::Variant newActionValue{5678};
+
+    this->m_proxy->actionVariant(newActionValue);
+
+    ASSERT_THAT(this->m_proxy->actionVariant().template get<int>(), Eq(5678));
+}