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.
Previously, we used a unique_ptr to a ModDownloadTask to keep track of
the selected mod to download when we accepted the dialog.
In order to allow multiple mods to be selected at once for download,
this has been changed to a QHash where the key is the mods name (since
it doesn't seem right to allow for multiple versions of the same mod to
be downloaded at once), and the value is a pointer to the corresponding
ModDownloadTask.
