ShellApp: Connect applications to systemd journal (if available)

Systemd-for-the-user-session would also do this, but that's a deeply
invasive change that I may not actually get to this cycle.  This
change is tiny and non-invasive, but provides an important benefit:
You can actually reliably tell *which* applications are logging which
messages (assuming they're launched by the shell).

This actually complements a recent change in DBus:
See https://bugs.freedesktop.org/show_bug.cgi?id=68559
which does a similar thing for bus activated apps.

https://bugzilla.gnome.org/show_bug.cgi?id=711626