Refactor update download logic, introducing a new 'task' class to
manager downloads, rather than trying everything through the network
manager's signals. Also, move the code to start such a task to a helper
function. Leverage these to hook up downloading of updates, now that
tasks can have different completion handlers assigned. Write update
archive file and scheduling information on update download completion.

Issue #2089

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23500 3bd1e089-480b-0410-8dfb-8563597acbee