Make silent on-demand update-checks available

Sorry if I'm bombarding you with PRs. This is a small one, I promise ;)

There might be cases where the application would want to check for
updates manually. This PR implements add the win_sparkle_check_update_without_ui function to the API which does exactly that. This function would override the users request to not perform
update checks, if that is enabled, so should be used with care. However, we have the ability to not let the users choose anyway so this might be fine.