Removed the library to ease the addition of providers.

[tintou]

Removed GSignond and replaced libgsignon-glib by libsignon-glib
Fixes #55 Fixes #44 Fixes #39

[tintou]

Will work once this is merged:
https://gitlab.com/accounts-sso/libsignon-glib/merge_requests/17

I'm going to continue to work on this so this branch is going to grow.

[vjr]

@tintou is it safe to remove an element from the list in the middle of a foreach?

                list.foreach ((nth) => {
                    if (nth.get_system_context () == path) {
                        list.remove (nth);
                    }
                });

[codygarver]

Looks fine now, just need somebody with accounts to prove the changes work. But I confirm it at least prompts for login.