New time scale `ticks.bounds` option (#4556)

`ticks.bounds` (`'data'`(default)|`'label'`): `data` preserves the data range while `labels` ensures that all labels are visible. This option is bypassed by the min/max time options.

Remove the useless time scale `_model` object containing private members: instead, make these members private (prefixed by `_`) part of the scale.