fix(diagram/header): fix how expanded height is computed
This fixes cases where line's height extent can be greater than the entire layout's height extent, causing difference in default and expanded height despite a maximum line count of 1 across layouts.
Requires https://gitlab.gnome.org/msandova/bustle/-/merge_requests/56 and https://gitlab.gnome.org/msandova/bustle/-/merge_requests/57
Edited by Dave Patrick