New time scale `ticks.mode/.source` options (#4507)

`ticks.source` (`'auto'`(default)|`'labels'`): `auto` generates "optimal" ticks based on min, max and a few more options (current `time` implementation`). `labels` generates ticks from the user given `data.labels` values (two additional trailing and leading ticks can be added if min and max are provided).

`ticks.mode` (`'linear'`(default)|`series`): `series` displays ticks at the same distance from each other, whatever the time value they represent, while `linear` displays them linearly in time: the distance between each tick represent the amount of time between their time values.