OSX: Fix showing window with no specified size.
OSX: Fix showing window with no specified size.

A Window without a `Width`/`Height` specified was not getting shown on OSX since 1f8b90925771c64e54afa1a264938de0fd5e3c70. Set the content view in the constructor to fix this.

AvaloniaUI/Avalonia,wieslawsoltes/Perspex,SuperJMN/Avalonia,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,wieslawsoltes/Perspex,jkoritzinsky/Avalonia,Perspex/Perspex,SuperJMN/Avalonia,jkoritzinsky/Avalonia,wieslawsoltes/Perspex,SuperJMN/Avalonia,jkoritzinsky/Perspex,jkoritzinsky/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,grokys/Perspex,grokys/Perspex,wieslawsoltes/Perspex,wieslawsoltes/Perspex,AvaloniaUI/Avalonia,wieslawsoltes/Perspex,SuperJMN/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,Perspex/Perspex,jkoritzinsky/Avalonia,wieslawsoltes/Perspex,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia