shell: Make KeyringPrompt.cancel() callable from signal handlers

gcr/gnome-keyring don't handle the case where prompt_password_async()/
prompt_confirm_async() return their result from within the function
very well (or rather: break badly). Calling gcr_prompt_close() instead
is safe, but to avoid these kind of errors in the future, modify
shell_keyring_prompt_cancel() to work as expected in this case.

https://bugzilla.gnome.org/show_bug.cgi?id=708187
keyring: Don't unregister the prompt when disabled

gnome-keyring provides a fallback in case our builtin prompt fails
to register, so keyring dialogs may still pop up even when they
are supposed to be disabled.
Instead, keep the prompt registered but cancel requests immediately
while disabled.

https://bugzilla.gnome.org/show_bug.cgi?id=708187
authPrompt: Clear _queryingService on verification failure

A conversation is finished after failing, and we are expecting a new
one to be started shortly after. However if we encounter an existing
reference to a previously set _queryingService, we will clear the
password entry, which might already contain a partially typed password
at that point. The behavior does make sense in the case of conflicting
conversations, but in the failure case it is both unexpected and
annoying, so clear _queryingService early to prevent this.

https://bugzilla.gnome.org/show_bug.cgi?id=708186
messageTray: Fix condition for adding the tray to CtrlAltTab

With the current condition, we wrongly add the message tray to
CtrlAltTab if we start up locked or as greeter.

https://bugzilla.gnome.org/show_bug.cgi?id=708380