desktop-entry-spec: define handling of terminal apps

Currently it's undefined how environments should handle Terminal=true and generally they just hardcode a list of known terminals

This isn't great as the user has no control over the precedence give to emulators (or even if an emulator is in the list)

This update aims to resolve this by building on existing handling of mimetypes

I've written an implementation for glib (specifically this commit), as you can see it's pretty trivial to implement