AK: Fix a race condition with WeakPtr<T>::strong_ref and destruction

Since RefPtr<T> decrements the ref counter to 0 and after that starts
destructing the object, there is a window where the ref count is 0
and the weak references have not been revoked.

Also change WeakLink to be able to obtain a strong reference
concurrently and block revoking instead, which should happen a lot
less often.

Fixes a problem observed in #4621

Libraries/LibGUI/Application.cpp

@@ -72,6 +72,11 @@ static NeverDestroyed<WeakPtr<Application>> s_the;
 
 Application* Application::the()
 {
+    // NOTE: If we don't explicitly call revoke_weak_ptrs() in the
+    // ~Application destructor, we would have to change this to
+    // return s_the->strong_ref().ptr();
+    // This is because this is using the unsafe operator*/operator->
+    // that do not have the ability to check the ref count!
     return *s_the;
 }