main: Merge the main header and fullscreen header

HdyFlap can act as a GtkBox or a GtkOverlay. We make full use of that so that the main header can act as the fullscreen header. With this, we avoid to have two separate header having the same content.

I just left the shadow overlay to get your opinion on whether to leave it or not. In the latter case, I would just add a CSS file like Epiphany.

Thanks