applets/systemtray: fix highlight being on top of first column items

By binding the content item's z to its x value, the first column where
x is 0 was being drawn underneath the highlight effect.

BUG: 456180


FIXED-IN: 5.27.6

co-authored-by: nttkde <watisthispoo@outlook.com>
(cherry picked from commit 4d828b1b)