System desktop files created for 'unity8' interface

[Ted Gould]

Apps that use the 'unity8' interface also have desktop files that are stored in /meta/gui/$(appname).desktop like 'unity7' and 'x11' applications. They are built for Mir and not X11 though, so they can't run in an environment that is X11 based. When an application that only has a unity8 interface is installed by snapd its desktop file is copied to /var/lib/snapd/desktop/applications and it gets picked up by Unity7 (and other X11 desktops). Though, when it runs, it exits because there is no Mir socket to connect to.

I think that desktop files should only be created for apps that have the 'unity7' or 'x11' interface in /var/lib/snapd/desktop/applications.