media-keys: Remove unused "needs_term" argument

https://bugzilla.gnome.org/show_bug.cgi?id=729202
media-keys: Set the keyring env for other launchers too

Instead of only applying the keyring environment to custom shortcuts,
also apply it to other applications we launch.

https://bugzilla.gnome.org/show_bug.cgi?id=729202
media-keys: Simplify launching custom commands

Instead of launching the app ourselves, create a new GAppInfo from the
custom command's string.

https://bugzilla.gnome.org/show_bug.cgi?id=729202