Make TaskManager independent of Future

`TaskManager` and `Future` where coupled together. When a task is pushed
a future is returned. Also the `Future::Wrap` function was inconvenient
to use.
Now `TaskManager::PushTask` doesn't return a `Future` anymore. This
alows to use different `Future`-like types.
Also it's possible to easily use the `Future` with a different type that
implements a `PushTask` function.

source/ps/TaskManager.cpp

@@ -185,7 +185,7 @@ size_t TaskManager::GetNumberOfWorkers() const
 	return m->m_Workers.size();
 }
 
-void TaskManager::DoPushTask(std::function<void()>&& task, TaskPriority priority)
+void TaskManager::PushTask(std::function<void()> task, TaskPriority priority)
 {
 	m->PushTask(std::move(task), priority);
 }