The different units - seconds for Tweener and milliseconds for
timeouts - are not a big issue currently, as there is little
overlap. However this will change when we start using Clutter's
own animation framework (which uses milliseconds as well), in
particular where constants are shared between modules.
In order to prepare for the transition, define all animation times
as milliseconds and adjust them when passing them to Tweener.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/663
476816732f