LibThreading: Remove Thread's inheritance from Core::EventReceiver

Inheritance from `EventReceiver` on the `Thread` class was only used in
the `BackgroundAction` class, where the children vector was keeping the
action alive until the work was completed. However, this can be
accomplished by instead capturing a `NonnullRefPtr` of `this`. The work
function can then avoid having to remove the `BackgroundAction` from
its parent `Thread` when the work completes.

Userland/Libraries/LibThreading/Thread.h

@@ -41,10 +41,19 @@ enum class ThreadState : u8 {
     Joined,
 };
 
-class Thread final : public Core::EventReceiver {
-    C_OBJECT(Thread);
-
+class Thread final
+    : public RefCounted<Thread>
+    , public Weakable<Thread> {
 public:
+    static NonnullRefPtr<Thread> construct(Function<intptr_t()> action, StringView thread_name = {})
+    {
+        return adopt_ref(*new Thread(move(action), thread_name));
+    }
+    static ErrorOr<NonnullRefPtr<Thread>> try_create(Function<intptr_t()> action, StringView thread_name = {})
+    {
+        return adopt_nonnull_ref_or_enomem(new (nothrow) Thread(move(action), thread_name));
+    }
+
     virtual ~Thread();
 
     ErrorOr<void> set_priority(int priority);