libxfce4ui user repo for gtk3

[Eduard Roccatello]

Hello folks,

I've started working on libxfce4ui porting to GTK3 as I've seen no-one 
is working on that yet (am I right?).

As usual I've setup my user repo on github
https://github.com/mastershadow/libxfce4ui

My last commit restricts compatibility to GTK3 3.14 or major.
I've kept the API versions that were set before when gtk3 was found (so 
it creates libxfce4ui-2.la and libxfce4kbd-3.la).

ciao!
Eduard