[ntcore] Revamp listeners (#4511)

- In both C++ and Java, add listener functions to Instance class (same as NT3 provided)
- Add WaitForListenerQueue functions (same as NT3 provided)
- Move Java non-poller implementation to Instance (previously only handled single instance)
- Change C++ listeners to take non-const references for subscribers etc to help avoid footguns from use of temporary objects (also add doc comment)
- Fix Preferences making .type persistent

ntcore/src/test/native/cpp/ConnectionListenerTest.cpp

@@ -89,12 +89,10 @@ TEST_P(ConnectionListenerVariantTest, Threaded) {
   wpi::mutex m;
   std::vector<nt::ConnectionNotification> result;
   auto handle = nt::AddConnectionListener(
-      server_inst,
-      [&](const nt::ConnectionNotification& event) {
+      server_inst, false, [&](const nt::ConnectionNotification& event) {
         std::scoped_lock lock{m};
         result.push_back(event);
-      },
-      false);
+      });
 
   // trigger a connect event
   Connect(GetParam().first, 0, 20001 + GetParam().second);
@@ -116,6 +114,9 @@ TEST_P(ConnectionListenerVariantTest, Threaded) {
   nt::StopClient(client_inst);
   std::this_thread::sleep_for(std::chrono::milliseconds(100));
 
+  // wait for thread
+  nt::WaitForConnectionListenerQueue(server_inst, 1.0);
+
   // get the event
   {
     std::scoped_lock lock{m};