replaced _gx_system_lock with GX_ENTER_CRITICAL

[parsley]

replaced _gx_system_lock and _gx_system_unlock with GX_ENTER_CRITICAL and GX_EXIT_CRITICAL.

This change mainly effects gx_horizontal_list_total_columns_set.c and gx_vertical_list_total_rows_set.c.
All other changes are only in comments.

[fdesbiens]

Thank you for this contribution, @parsley!

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