Merge lp://staging/~ted/ubuntu-app-launch/lp1495871-unref-context into lp://staging/ubuntu-app-launch/16.04

Ensure all pending events on the context are complete before unref'ing it.

So while we create a context to ensure that we can capture all the events to the CGManager connection it seems that some of those events can get stuck on the context, and they need to get processed to complete. Otherwise they hold onto references of things, and that actually results in a g_spawn getting left around because it never gets free'd. (actually of the context itself, which is a bit confusing, but eh, refcounting is fun)

Also brought in the pid tools branch because it made testing this much, much easier. You can use the ubuntu-app-list-pids tool to cause the FD to be leaked and then see it fixed with something like (assuming you've done ubuntu-app-launch firefox):

valgrind --track-fds=yes ./ubuntu-app-list-pids firefox

Which on trunk will have 6 FDs left over, but with this branch only 5. (which come from globals that we don't control)