This puts all mod downloading tasks inside a SequentialTask, which is,
for more than one task, a multi step task. This is handled by the
ProgressDialog by showing both the global progress of tasks executed,
and the individual progress of each of them.
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.
