ellipsize overlong icon texts and add tooltips, bnc#879959

For the overlong text of icons in yast2-control-certer-gnome, ellipsize them and add mouse hovering tooltips.
Hi Josef, thanks so much for your review.
The rest of the code uses 2 whitespaces as indentation so I thought I should keep consistent to it.
Or please let me know if I misunderstood your comment. Thanks!