Cancel Modpack Download

[FyberOptic]

I occasionally and accidentally double-click on a pack which I don't have installed, which naturally starts trying to download it, and unfortunately there's no way to cancel this process. Worse, if you close the download dialog, the launcher remains in an unusable state, possibly until the pack finishes downloading in the background (since I don't believe closing the dialog kills the download task, unbeknownst to the user).

This may or may not be wanted, but I added a cancel button to the download dialog to kill the worker process. Closing the dialog does this now as well. The pack download is then perceived as having failed, and the launcher returns to a usable state. Oh, and I also made the dialog center itself, like I did with the main window.

I'll do the PR if this is acceptable behavior to implement: 8612734