Revert flat headerbar
The mockups https://gitlab.gnome.org/Teams/Design/app-mockups/-/blob/master/tour/tour-headerbarless.png use a flat headerbar, but that would require setting a window handle over the entire window. This is a no-no as the handle steals the input from the carousel and makes swipe navigation impossible.
I think the best solution is to remove the flat style but maybe there is another way.
Note that the flat style will be used after !60 (merged) lands.
cc: @exalm @bertob
Edited by Maximiliano