Fix shutdown on Revit.

Move update manager event handlers off controller to view model.
Provide additional Exit method that allows you to shutdown the host
application as well.
Save last update download location.

Placeholder for future optimization. On startup, we should automatically
show the update notification if an update was downloaded but never
applied. Currently we will check for a new update regardless of whether
there is one on disk already.
Add a tool tip to the update notification control.
Update manager refactoring.

Show download progress while downloading.
Refactor main version checking method. Better handling of daily builds
and stable builds.