UI Freeze Exception: Add system-config-printer-applet to systray whitelist

[Jeremy Bicha]

I have reported http://pad.lv/771562 which requests that
system-config-printer-applet by added to Unity's systray whitelist. (
http://pad.lv/588200 is the bug about this needing to be converted to
an appindicator)

In 10.10 and previous releases, the applet shows when a job is sent to
the printer and gives the user easy access to pause, cancel, or view
print jobs. Without this, the user would need to open the Dash then
open Printing. Right click on the printer and select View Print Queue.
That is quite a few steps for something that used to be easy to
access.

The docs actually already assume that print jobs can be canceled by
clicking the print icon in the panel. So we'd have to fix the docs if
this change /didn't/ happen.

Jeremy Bicha