Add iterators for various iterables

We want to support the following API.

vtkProxyManager

vtkSMProxyManager* pxm = ...

/// iterate over all registered proxies
for (const auto& el : pxm->GetRegisteredProxies())
{
   const std::string registrationName = el.GetName();
   const std::string registrationGroup = el.GetGroup();
   vtkSmartPointer<vtkSMProxy> proxy = el.GetProxy();
   ...
}

/// iterate over one registration group
for (const auto& el : pxm->GetRegisteredProxies(groupName))
{
   const std::string registrationName = el.GetName();
   const std::string registrationGroup = el.GetGroup();
   vtkSmartPointer<vtkSMProxy> proxy = el.GetProxy();
   ...
}

/// iterate over all prototype proxies
for (const auto& el : pxm->GetPrototypeProxies())
{
   const std::string xmlname = el.GetName();
   const std::string xmlgroup = el.GetGroup();
   vtkSmartPointer<vtkSMProxy> proxy = el.GetProxy();
   ...
}

/// iterate over one group of prototype proxies
/// here the group  name is the definition group of the prototypes
for (const auto& el : pxm->GetPrototypeProxies(groupName))
{
   const std::string xmlname = el.GetName();
   const std::string xmlgroup = el.GetGroup();
   vtkSmartPointer<vtkSMProxy> proxy = el.GetProxy();
   ...
}

vtkSMProxy

vtkSMProxy* proxy = ...

// iterate over all properties
for (const auto& el : proxy->GetProperties())
{
  auto name = el.GetName();
  auto property = el.GetProperty();
  ...
}

// iterate over all properties in order of definitions
for (const auto& el : proxy->GetOrderedProperties())
{
  auto name = el.GetName();
  auto property = el.GetProperty();
  ...
}

vtkSMProperty

vtkSMProperty* property = ...
for (const auto& el : property->GetDomains())
{
   auto name = el.GetName();
   auto domain = el.GetDomain();
}