OverlayDrawer: Fix bugs and rework separator's code, animate transitions

Bind segmented separator's width to the real separator's width

Just in case, because who knows.

Rewrite segmented separator positioning expression

Specifically, rewrite it in a way that does not require math on NaN and undefined values.

Rewrite visibility condition for segmented separator

I found one-liner quite confusing to read, not to mention it was relying to on the && and || operator precedence without parenthesis.

There is no need to check for hasOwnProperty(), because instanceof check would evaluate to false with null or undefined value on the left.

Hide segmented separator when the drawer is collapsed

Collapsed drawer have a compatible header but it's hidden above. The segmented separator looks weird in that case. See Emoji Selector as an example.

Rework separator's code, animate transitions

The main point is to use a real second Separator item instead of hacking the visuals with a patch of a background-colored rectangle.