C++ NetworkTable: Provide AddSubTableListener().

This matches the implementation provided by Java NetworkTable, with the
exception that the subtable is not provided (because it's not a value).
The listener still can get access to the subtable by calling
source->getSubTable(key).

src/networktables/NetworkTable.cpp

@@ -3,6 +3,7 @@
 #include <algorithm>
 
 #include "llvm/SmallString.h"
+#include "llvm/StringMap.h"
 #include "tables/ITableListener.h"
 #include "ntcore.h"
 
@@ -136,6 +137,33 @@ void NetworkTable::AddTableListener(StringRef key,
   m_listeners.emplace_back(listener, id);
 }
 
+void NetworkTable::AddSubTableListener(ITableListener* listener) {
+  std::lock_guard<std::mutex> lock(m_mutex);
+  llvm::SmallString<128> path(m_path);
+  path += PATH_SEPARATOR_CHAR;
+  std::size_t prefix_len = path.size();
+
+  // The lambda needs to be copyable, but StringMap is not, so use
+  // a shared_ptr to it.
+  auto notified_tables = std::make_shared<llvm::StringMap<char>>();
+
+  unsigned int id = nt::AddEntryListener(
+      path,
+      [=](unsigned int uid, StringRef name, std::shared_ptr<nt::Value> value,
+          bool is_new) mutable {
+        StringRef relative_key = name.substr(prefix_len);
+        auto end_sub_table = relative_key.find(PATH_SEPARATOR_CHAR);
+        if (end_sub_table == StringRef::npos) return;
+        StringRef sub_table_key = relative_key.substr(0, end_sub_table);
+        if (notified_tables->find(sub_table_key) == notified_tables->end())
+          return;
+        notified_tables->insert(std::make_pair(sub_table_key, '\0'));
+        listener->ValueChanged(this, sub_table_key, nullptr, true);
+      },
+      true);
+  m_listeners.emplace_back(listener, id);
+}
+
 void NetworkTable::RemoveTableListener(ITableListener* listener) {
   std::lock_guard<std::mutex> lock(m_mutex);
   auto matches_begin =