app: Restore docks in an idle handler

In single-window mode, gimp_session_managed_set_aux_info() will set
the size of the dock areas at the sides. If we don't wait for those
areas to get their size-allocation, we can't properly restore the
docks inside them, so do that in an idle callback.

Also add a required call to gimp_test_run_mainloop_until_idle()
because of this in the 'automatic_tab_style' test.