Merge lp://staging/~mterry/libindicator/rename-unit into lp://staging/libindicator

Use indicators.target as the systemd lifecycle unit, not indicators-pre.target.

This MP might just be my inexperience with systemd conventions, but I think "indicators.target" makes more sense than "indicators-pre.target"?

With this MP, indicators can do the following:

[Unit]
PartOf=indicators.target
[Install]
WantedBy=indicators.target

And then they will start, restart, and stop when indicators.target is. And unity8 can simply say Wants=indicators.target when it starts up (or manually start it, whatever).

Indicators don't seem to be started by anything now, but I plan to add the above stanza to indicators if this branch is liked.

Is indicators-pre.target separately valuable? The "-pre" stuff seems used elsewhere (like in graphical-session), but I'm not sure why we want it for indicators.