CamelKolabSession: implemented alert_user() virtual function
CamelKolabSession: implemented alert_user() virtual function

* implemented this one since the CamelSession does
  not (and cannot) implement a template for it
* in Evo context, this function will pop up a dialog,
  present a prompt string and (possibly) ask for a
  user input
* we cannot open a dialog from within our backend
  context, since it runs in E-D-S, which itself does
  not link against an UI toolkit
* so we just print the prompt message as a warning
  to the console and move on
* in case this function is called when a server SSL
  certificate is to be accepted/denied, a dialog for
  the same will be opened in Evo and the answer given
  there by the user will be picked up by our backends
  on next E-D-S restart

GNOME/evolution-kolab,GNOME/evolution-kolab