Prevents a massive inload of Scrumplex ditto's :)
I didn't add it to every file modified in this PR because the other
changes are pretty minor, and would explode the diff of the PR. I hope
that's not a problem O_O
This makes ModDownloadTask into a SequentialTask with 2 subtasks:
Downloading the mod files and updating the index with the new
information.
The index updating is done first so that, in the future, we
can prompt the user before download if, for instance, we discover
there's another version already installed.
