Upstream parent window support

[Ms2ger]

<tomaka> Ms2ger: there's a minor change to make to glutin before deforking
<tomaka> it would be great if someone did it :o)
<Ms2ger> Ptr?
<tomaka> the servo fork added a `with_parent_window` method to the WindowBuilder or something like that
<tomaka> and a `parent_window` member
<tomaka> these are not yet in glutin
<Ms2ger> And you'd take those?
<tomaka> Not as it is in servo
<tomaka> the parent_window thing is specific to Linux
<tomaka> so the `parent_window` member should be put in this struct: https://github.com/tomaka/glutin/blob/master/src/platform/linux/mod.rs#L19
<tomaka> and `with_parent_window` is there: https://github.com/tomaka/glutin/blob/master/src/os/unix.rs#L44
<tomaka> and that's it
<tomaka> obviously the linux code needs a few small fixes as well to adjust to that, but they should be minor
<tomaka> also the name should make it obvious that it's specific to xlib and not wayland
<tomaka> so not just "parent_window" but "xlib_parent_window" or something