LibCore: Add CThread, a simple thread abstraction object.

Currently this is only a simple wrapper around create_thread() that
remembers the thread ID of the spawned thread.

Libraries/LibCore/CThread.h

@@ -0,0 +1,18 @@
+#pragma once
+
+class CThread {
+public:
+    static CThread& main_thread();
+
+    CThread(int (*entry)(void*), void* user_data);
+    ~CThread();
+
+    bool is_main_thread() const { return m_thread_id == 0; }
+    int thread_id() const { return m_thread_id; }
+
+private:
+    enum MainThreadTag { MainThread };
+    explicit CThread(MainThreadTag);
+
+    int m_thread_id { -1 };
+};