This makes it possible to run a task in another thread.
I added a variable to toggle debug prints because they seem to trigger
an assertion on Qt internals when the task in on another thread. Of
course, this isn't awesome, but can wait until we improve our logging.
Signed-off-by: flow <flowlnlnln@gmail.com>
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.
