gdesktopappinfo: Add support for MATE and Xfce4 terminals
Add support for mate-terminal and xfce4-terminal with higher precedence
over xterm as it's likely people that have those want to use them.
They both use the gnome-terminal
-x switch semantics instead of xterm's
I know there is #338, but this doesn't look like it's not gonna be solved anytime soon, and the approach here while not great should give a better experience for MATE and Xfce users at very low cost until a grand solution is found for this.