bug#23066: "guix package -i transmission" doesn't install transmission-gtk into profile

[Danny Milosavljevic]

However, it installs share/applications/transmission-gtk.desktop into the 
profile, containing

Exec=transmission-gtk %U

i.e. it won't find it.

transmission-cli works.

I checked gnu/packages/bittorrent.scm , it seems to do some weird special-case 
for the gui (it puts the gui into a "gui" output and the rest into an "out" 
output).

But

$ guix package -i transmission:gui

works. If that's supposed to be the official way, that's not discoverable. 

Also, the desktop file should also be in there, then it's at more consistent.