Show the display name of launchable desktop files

When displaying a desktop file that we'll launch (executable), we currently show the filename. But desktop files come with a name - let's show that. We don't want to reopen CVE-2017-14604 though, so only do this for files with `metadata::trusted' set to 'true'. Future work would be to consider a UI for this, or an alternative solution.

e.g. on Ubuntu's live images we put a desktop file on the desktop which launches the installer. It'd suck if this was shown as ubiquity.desktop instead of "Install Ubuntu".