Fix #349. Now the command "wall" work properly.

The bug comes from a poor implementation of VTE in GTK2.
This bug does not exist in GTK3.

Guake now has a dependency on the library libutempter.
Hello,

"Please refere this dependency in the README"
> Done

"If the library is not found, guake should not crash. Set libutempter to None in this case"
> Done.

"please add 2 lines here (use ./validate.sh)"
> What ?
I have tried to run your script but it fail. (like https://travis-ci.org/Guake/guake/builds/39791585)