Parent window abstraction.
The [struct@Parent] struct provides an abstract way to represent a window, without introducing a dependency on a toolkit library.
An XdpParent implementation for GTK is included in the `portal-gtk3.h` and `portal-gtk4.h` header files, in the form of inline functions. To create a XdpParent for a GTK window, use `xdp_parent_new_gtk()`.