app: keep docks on the monitor in gimp_ui_configurer_move_docks_to_window()

and don't move them to the very left and right of the screen, which might
be on different monitors. Also clean up the function.
app: make sure the GUI appears on the same monitor as the splash

Manually figure the monitor where the pointer is and pass it to the
splash, the empty image window and to session_restore() explicly.