Currently the separators in headers are drawn at the right edge for each
section. That makes it visible on the rightmost edge of a header, directly
adjacent to the surrounding frame, surrounding in a 2px wide line.
(For RTL it's the reverse)

Avoid the double line by moving the separator to the adjacent "HeaderEmptyArea"
instead, which is only visible when necessary.

As a side effect, this commit fixes the check for whether a section is "last"
for invisible headers and RTL layouts.