Some callers of the keyring prompt keep the dialog up while
processing the prompt. Allow the user to cancel the prompt
while in this state.
This is propagated to the caller, who can cancel the operation
in question when this occurs.
https://bugzilla.gnome.org/show_bug.cgi?id=682830
* Add a keyring prompter based on GcrSystemPrompter
* Adds dependency on gcr version 3.3.5 or higher
* Not yet using unmerged support for non-pageable memory
https://bugzilla.gnome.org/show_bug.cgi?id=652459