Provide a configuration to ignore certain properties from the Builder pattern

[bilelmoussaoui]

There's a use case, in Adw::Window, where you can still call .child(some_child) and it should be usable; instead you're supposed to call .content(some_child)

Having the other property auto-generated doesn't help.

[bilelmoussaoui]

Before we can fix this one, we would have to wait until #1417 lands first as otherwise we would have to configure every child widget to ignore a property that comes from a parent widget.