Allow usage of HTTP Upload without sending a message

In the case that a plugin wants to utilise Gajim's implementation of HTTP Upload without sending a message with the link, the plugin has to reimplement almost everything that nbxmpp does not provide.

As such, I added a "upload done" callback to check_file_before_transfer. For the GUI interactions, I added a "check_file_before_transfer_gui" that just does what it did before.

EDIT: I have now used this patch for a while and noticed no issues with it. Additionally, I now added a new parameter to check_file_before_transfer that allows to not show the progress window.

Also, something must have broken while I tried to rebase it with master. I'll take a look