Implement Send for Display and Window

[Mylloon]

Fix #145

Inspired from nashaofu/xcap#231

This allow to do:

let mut capturer = Capturer::build(Options {/* ... */})?;

// Start capturing
capturer.start_capture();

// Consume all our frames to prevent a memory explosion
let frame_clone = Arc::clone(&old_frame);
spawn(move || {
    capturer; // !!! This move isn't allowed on Windows without Send trait
});