[wip] gigolo port to gtk3

[Landry Breuil]

Hi,

this evening i wanted to do more porting to gtk3.. and gigolo was
waiting for me. I have to admit i never looked at the code before, but
it's very well structured, and it's been quite easy to get it to a state
where it builds and basically runs.

There's still of course lots of deprecated symbols still in use, and i
need to properly fix/do the GtkApplication unique dance, and waaayyy
more testing, but so that the work isnt lost, i've pushed it to a user
repo at http://git.xfce.org/users/landry/gigolo/

Enrico, if you feel like giving it a shot..
I've updated https://wiki.xfce.org/releng/4.14/roadmap#applications.

Landry