* app/core/gimpgrid.[ch]: derive GimpGrid from GimpObject (not
GObject) allowing us to use gimp_object_get_memsize(). Added
missing copyright notice.

* app/core/gimpimage.c: only call gimp_object_get_memsize() if
gimage->grid != NULL.

* app/gui/grid-dialog.c: added mnemonics, set shell->grid_dialog
to NULL when destroyed.

* app/gui/view-commands.c (view_configure_grid_cmd_callback):
added call to gtk_window_present().