More clear scoping and interaction with running actions in
UmAccountDialog. In later 'enterprise login' patches we have long
running actions that's why this needs cleaning up.
In particular:
* Show errors as children of the dialog.
* Errors don't make the account dialog go away, user can correct
problems.
* Use more standard GAsyncResult style callbacks:
um_account_dialog_perform() um_account_dialog_finish()
* Disable controls while the operation is happening.
* Allow the user to cancel long actions in UmAccountDialog by
pressing the cancel button.
https://bugzilla.gnome.org/show_bug.cgi?id=677548
We were leaking various references, leading to failures to populate
the list upon reloading, or to segfaults. With this commit, all
created user objects, as well as the user-manager object are
finalized when the panel is unloaded.
