Core: Cleanup headers in core/config

- `MainLoop` now forward-declared in `OS`

core/os/main_loop.h

@@ -30,9 +30,8 @@
 
 #pragma once
 
-#include "core/input/input_event.h"
 #include "core/object/gdvirtual.gen.inc"
-#include "core/object/ref_counted.h"
+#include "core/object/object.h"
 
 class MainLoop : public Object {
 	GDCLASS(MainLoop, Object);

core/os/os.h

@@ -40,6 +40,8 @@
 
 #include <cstdlib>
 
+class MainLoop;
+
 class OS {
 	static OS *singleton;
 	static uint64_t target_ticks;
@@ -223,6 +225,7 @@ public:
 
 	void ensure_user_data_dir();
 
+	// NOTE: MainLoop is forward-declared in OS and should be included to use this.
 	virtual MainLoop *get_main_loop() const = 0;
 
 	virtual void yield();