Memory leak when closing window

[dathinaios]

As reported at robbert-vdh/nih-plug#173. I can reproduce on a Mac on Bitwig and Reaper on a nih_plug plugin (parented window).

The following addition on WindowInner's close function for Mac resolves the issue for me on Mac:

// Ensure all subviews are detached and released
let subviews: id = msg_send![self.ns_view, subviews];
let count: usize = msg_send![subviews, count];
for i in 0..count {
    let subview: id = msg_send![subviews, objectAtIndex: i];
    subview.removeFromSuperview();
    let () = msg_send![subview, release];
}

A similar change need to happen on Windows and Linux.