gcr: Implement prompt-close in GcrSystemPrompt and GcrSystemPrompter

gcr: Implement prompt-close in GcrSystemPrompt and GcrSystemPrompter

 * Properly relay the prompt-close signal from GcrSystemPrompter
   back to GcrSystemPrompt by firing the PromptDone callback method
   on the caller's DBus interface.
 * Make sure GcrSystemPrompt emits prompt-close appropriately for
   all the various paths that can close the prompt.
 * Add testing of the above, and changes in the mock prompter for this.

https://bugzilla.gnome.org/show_bug.cgi?id=678611