widget: Give some meaning to "visible"

widget: Give some meaning to "visible"

GtkWidget::visible is required for the widget to:
- have a preferred size other than 0/0
- have a size allocated
- return other values than { -1, -1, 1, 1 } from get_allocation()

This is an experimental patch aiming to make concepts and behaviors
inside GTK more concreate. GtkWidget::visible is now essentially what
CSS does for "display: none".

Note that if you want the effect of CSS's "visibility: hidden", you'll
have to use a GtkNotebook with an empty page as the concept of reserving
space but not drawing anything isn't supported natively in GTK.