Sort out .titlebar for in-window headerbars
Normally, the widget you set as window titlebar has .titlebar
style class. Adwaita uses it for some things, such as .devel
style.
However, with HdyWindow
one is expected to put the headerbar inside the window and it won't get .titlebar
. We need to either add it automatically for HdyHeaderBar
or otherwise do something about it.