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.
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