Add a titlebar widget
Animating widgets in the title bar is broken in GTK+ because of the rounded corners: https://gitlab.gnome.org/GNOME/gtk/issues/1264. It is a problem as we want header bars into animated leaflets into as titlebars for many adaptive applications.
Waitinig for a fix upstream doesn't seem like a viable solution time-wise, so to work around this we could write a widget responsible for drawing the titlebar style and doing nothing else, in which we could safely do animations.