give mate-terminal a unique name and allow to display them in other DE's

a) avoid name overlaps with gnome-terminal and xfce's terminal displayed in menu and default-apps.
b) in my opinion we should allow to display mate-terminal in other desktop enviroments.
I know from a user who used m-t in fedora xfce, see
https://bugzilla.redhat.com/show_bug.cgi?id=908105

Note: po files need to be updated too