Want Error::steal() -> *mut GError, or a general way to steal() boxed types about gtk3-rs

[sdroege]

Naming in std for such things is into_raw(). We could call it into_glib_ptr() maybe, that would seem most consistent with both std and glib.

[sdroege]

It also makes some sense for Shared<T>: it would prevent the unneeded ref() from to_glib_full() that is directly followed by a unref() again from Drop::drop() of the original value.

[sdroege]

We also currently work around this for GValue in the get_property trampoline for subclassing: https://github.com/gtk-rs/glib/blob/63b5499ef3f2198b31e73141ddb0e42dc945aa42/src/subclass/object.rs#L95-L106

[sdroege]

As this happens very often now, I'd suggest simply adding an fn glib::Error::into_raw(self) -> *mut glib::ffi::GError like we already have for glib::Value.