Make window centered

[Kagami]

Would be nice to have ability to center window created by glutin. By default or with additional function call. Or at least to have ability to specify window's (x, y) position.

Seems like in OS X implementation windows are centered by default: https://github.com/tomaka/glutin/blob/119bd6393f5f3b8e1510ba6ab3281c8fb387449f/src/api/cocoa/mod.rs#L451

[Kagami]

Oh, there is actually set_position function. Don't know how I was able to miss it.
So it's in fact possible to center the window with glutin, you just need to calculate the coords by yourself.

[fkaa]

The documentation for NSWindow::center states:

The window is placed exactly in the center horizontally and somewhat above center vertically. Such a placement carries a certain visual immediacy and importance. [...]

You typically use this method to place a window—most likely an alert dialog—where the user can’t miss it. [...]

We should probably manually calculate the position to make it exact.

[tomaka]

Closing for rust-windowing/winit#38