Temporarily stop authentication on VT switch or screen blank

[kitsunyan]

Hi!

This patch allows to cancel authentication process on VT switch (Ctrl+Alt+Fx) or DPMS screen blank (I have configured xset dpms 0 0 15 in display-setup script). It will be resumed after switching VT back or screen unblank.

This might be helpful for those who use fingerprint reader for authentication. Fingerprint reader is activated by pam module and it remains taken over until authentication process ends. This leads to 2 problems:

1. If I switch VT and try to authenticate there, I won't be able to use fingerprint reader for this.
2. Fingerprint reader's LED is glowing all this time, which is extremely annoying. Of course the reader is active all this time, so it consumes power as well when session is locked.

This feature is optional and should be configured via new stop-on-inactivity configuration option which is disabled by default.

Mini-Greeter will require 2 additional libs: X11 (which actually isn't implied by gtk+-3.0) and Xss (allows to monitor screen blanking).