I pushed a few EShell features up to GtkApplication for GTK+ 3.2,
so we can now trim off the redundancies in EShell.

1) GtkApplication has a new "window-added" signal which replaces
   EShell's own "window-created" signal.

2) GtkApplication has a new "window-removed" signal which replaces
   EShell's own "window-destroyed" signal.

3) gtk_application_get_windows() now returns a list of windows sorted
   by most recently focused, replacing e_shell_get_watched_windows().

4) GtkApplication now provides enough hooks to subclasses that we can
   remove e_shell_watch_window() and call gtk_application_add_window()
   directly.