Improvements to the "Download from Webservice" dialog

- Added a help button that links to the online API page and should provide
a sufficient start to the new user.
- Use download_dialog_response_cb as a callback for most dialog reponses
instead of waiting for gtk_dialog_run() and using goto e.g. in the case
of GTK_RESPONSE_HELP.
- Fixed some integer signedness warnings in download_dialog_traverse_xml()

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>