Remove minidialog list from utils.

  It's supposed to be used to delete them when the connection is dropped, but this list is never added to. And the dialogs are automatically closed when the account disconnects in some other way that I could not find.

  Consequently, drop the unused gc from pidgin_make_mini_dialog*.
Replace PidginUtilMiniDialogCallback by PidginMiniDialogCallback.

  It is just a wrapper around the latter, with different argument order.
Move minidialog wrappers from utils to minidialog.c. And fix naming/arguments to match the existing functions.

Compiled, connected an account that had buddy requests, but didn't act on them.