application: don't add a custom icon search path

We don't install icons there anymore.
application: drop load/save of custom accel maps

We're moving towards a GAction-based model, and gtk-can-change-accels is
ignored in GtkSettings anyway. Just drop this.
application-actions: remove unneeded code
application: cleanup use of gtk_application_get_windows()

We're really only interested in the NautilusWindows that we create - so
keep track of them explicitly instead of checking for
NAUTILUS_IS_WINDOW() every time.