Userland: Rename Core::Object to Core::EventReceiver

This is a more precise description of what this class actually does.

Userland/Libraries/LibThreading/Thread.cpp

@@ -12,7 +12,7 @@
 namespace Threading {
 
 Thread::Thread(Function<intptr_t()> action, StringView thread_name)
-    : Core::Object(nullptr)
+    : Core::EventReceiver(nullptr)
     , m_action(move(action))
     , m_thread_name(thread_name.is_null() ? ""sv : thread_name)
 {