This patch combines several high level changes which are conceptually
independent but in practice rather intertwined.

* Add a "state" property to ShellApp which reflects whether it's
  stopped, starting, or started.  This will allow us to later clean
  up all the callers that are using ".get_windows().length > 0" as
  a proxy for this property
* Replace shell_app_launch with shell_app_activate and shell_app_open_new_window
  A lot of code was calling .launch, but it's signficantly clearer
  if we call this ".open_new_window()", and later if we gain the ability
  to call into an application's menu, we can implement this correctly rather
  than trying to update all .launch callers.
* Because ShellApp now has a "starting" state, rebase panel.js on top of
  this so that when we get a startup-notification sequence for an app
  and transition it to starting, it becomes the focus app, and panel.js
  cleanly just tracks the focus app, rather than bouncing between SN
  sequences.  This removes display of non-app startup sequences, which
  I consider an acceptable action in light of the committed changes
  to startup-notification and GTK+.

https://bugzilla.gnome.org/show_bug.cgi?id=614755