Suspending/Terminating a downloading task safely

[CodingEric]

The current version of the library does not offer an internal method to suspend/terminate a download task SAFELY, and workarounds on the linked launcher that do not modify the core's code are almost impossible to suspend/terminate the tasks safely for they are asynchronous and terminating them from outside results in exceptions. Whereas the function to suspend or terminate a task is severely in need especially for launchers that aim to be user-friendly considering that some users might regret their actions to download/launch a game.

Though this is an issue from a developer, we may not handle it in a short period. So PRs are welcome :)